I've been experimenting with various manual mans in Gimp of colorizing grayscale/black & white images to where it would naturally have looked to the eyes of the person who photographed the grayscale or black & white image. Several experimental attempts have resulted with with limited success,but apparently I I found a new manual means of colorizing such mentioned images with promising results to achieve the goal of what is mentioned & also give anyone the idea to write code that would speed up the process & make it easier. I first made an image to be used as a tool to help this process using rows of various basic gradients that cover as follows:Grayscale of black (0) to white (255) Hue of 0 to 360 degrees, Hue with value (red,green,blue,cyan,magenta,yellow),Hue with saturation (red,green,blue,cyan,magenta,yellow) The image needs to be zoomed to 800 or 1600 percent in order to use this image & a grayscale duplicate of this image needs to be pasted as a layer without merging the layers in order to properly use the image as a tool. First of all, you need to make two duplicates of this image & close the original,then convert the second duplicate to grayscale & copy then paste it as a layer into the first duplicate.

I've been experimenting with various manual mans in Gimp of colorizing grayscale/black & white images to where it would naturally have looked to the eyes of the person who photographed the grayscale or black & white image.
Several experimental attempts have resulted with with limited success,but apparently I I found a new manual means of colorizing such mentioned images with promising results to achieve the goal of what is mentioned & also give anyone the idea to write code that would speed up the process & make it easier. I first made an image to be used as a tool to help this process using rows of various basic gradients that cover as follows:Grayscale of black (0) to white (255) Hue of 0 to 360 degrees, Hue with value (red,green,blue,cyan,magenta,yellow),Hue with saturation (red,green,blue,cyan,magenta,yellow) The image needs to be zoomed to 800 or 1600 percent in order to use this image & a grayscale duplicate of this image needs to be pasted as a layer without merging the layers in order to properly use the image as a tool. First of all, you need to make two duplicates of this image & close the original,then convert the second duplicate to grayscale & copy then paste it as a layer into the first duplicate.

Next Steps:The next steps are to take an original grayscale or black & white image,make three duplicates of it & close the original,then convert the three duplicate images to grayscale then to RGB.

Each of the three images are to be used, one for each of the three R,G,B or C,M,Y colors.

I've written a description of how for example the R,G,B colors & their channel values are arranged in order to colorize a grayscale or black & white image as follows,but can be used with C,M,Y in place of R,G,B along with the examples of grayscale (K) values of each grayscale pixel. I'm currently using GIMP 2.10.2 using the Select by Color tool & the color Picker tool set to use info window. (Use Info window checked in color picker tool) With Gimp 2.10.2,being used, when threshold in select by color tool is set to 0.0,some of the pixels won't select as they should other than in 2.10.0 or latest version of 2.8 of Gimp. Which means unless this bug is fixed,I have to set the threshold of the Select by Color tool up to 1.0 or more & I haven't yet reported this bug in 2.10.2 with the Select by Color tool.

Okay,now with the following description as promised:

Grayscale (K) values of grayscale pixels for example: K:188,K:205,K:230.

R,G,B (RGB across top for color of pixels to be colorized)

R: G:
B: (Channel values of colorized pixels from top to bottom)

Example: R G B
R:255,0, 228
G:162,235,228
B:162,0,255

From top to bottom: R color equivalent of K of 188: R:255,G:162,B:162 G equivalent of K of 205: R:0,G:235,B:0 B equivalent of K of 230: R:228,G:228,B:255

Example of how to colorize the pixels in the three separate images for red, green & blue:(using Bucket Fill tool & Change Foreground Color)

For example in the image for red along with the horizontal & vertical position numbers for the pixels:0,0: R:255 with green & blue in Change Foreground Color set to 0 for example.(255 meaning channel value with 0,0 meaning horizontal & vertical positions of pixel for example) 0,1:R :0. 0,2:R:228. In image for green: 0,0:G:0. 0,1:G:235. 0,2:G:255. In image for blue:0,0:B:228. 0,1:B:228. 0,2:B:255.

Two ways to preview the resulting colorized image: Copy & paste blue & green images into red image as layers, set background (red) image as Normal,& green & blue images as Screen in Layers dialog window.

Second method of previewing resulting colorized image:(Decompose to RGB or CMY Decompose all three red,green & blue images into layers or each mentioned red,green & blue or cyan,magenta & yellow image to three separate images. Select in Compose to RGB or CMY three out of the nine separate images or layers as follows for example:Example using Decompose to RGB & Compose to RGB:red-6/red-14, red-7/green-19,red-8/blue-24.

Before doing the above steps,use the image I included in my post to use as a tool as follows:Download & open the image that I described & posted above & make two duplicates,close original & convert second duplicate to grayscale then copy & paste grayscale converted second duplicate as layer into first duplicate. Do not merge the layers.

Leave the grayscale converted second duplicate of the tool image open in case you lose what you copied & pasted or if you make a mistake in the grayscale layer,select Undo in the grayscale layer. Zoom the image to 800 or 1600 percent. Move the slider on the right all the way up. Now choose the grayscale pixel of the image you want to colorize. Set threshold in Select by Color tool to 0.0. Use Composite in Select by Color & select in Layers window the grayscale layer of the tool image. Click Color Picker tool twice & uncheck sample average & sample merged & check Use info window.
Click the grayscale pixel you want to colorize with the Color Picker tool & the grayscale value should appear in the info window. Using the grayscale value that appears for example 188,select the grayscale layer of the tool image. At the top horizontal row at 0,use the bottom slider to move the image left or right until the color picker tool displays in the info window the grayscale value (example 188). For example,the grayscale value of 188 will be at the position of x:188,y:0 as displayed in the info window. Click this area with the Select by Color tool. If using Gimp 2.10.2 increase the threshold any where up to 1.0. (I typed in 0.1 in the threshold slider.) Next,turn off the eye icon in the grayscale layer in the tool image & select bottom (background) layer. Move the bottom image slider left or right until the selected area appears. For example,I moved the slider to the left until the selected area at 93,3 for red appeared,meaning the horizontal & vertical positions of that area. Click on that area with the Color Picker tool. The RGB channel values should appear in the info window.

Use what I described above for the rest of the colorizing procedure. Any helpful replies or comments are appreciated.

Next Steps:The next steps are to take an original grayscale or black & white image,make three duplicates of it & close the original,then convert the three duplicate images to grayscale then to RGB.

Each of the three images are to be used, one for each of the three R,G,B or C,M,Y colors.

I've written a description of how for example the R,G,B colors & their channel values are arranged in order to colorize a grayscale or black & white image as follows,but can be used with C,M,Y in place of R,G,B along with the examples of grayscale (K) values of each grayscale pixel. I'm currently using GIMP 2.10.2 using the Select by Color tool & the color Picker tool set to use info window. (Use Info window checked in color picker tool) With Gimp 2.10.2,being used, when threshold in select by color tool is set to 0.0,some of the pixels won't select as they should other than in 2.10.0 or latest version of 2.8 of Gimp. Which means unless this bug is fixed,I have to set the threshold of the Select by Color tool up to 1.0 or more & I haven't yet reported this bug in 2.10.2 with the Select by Color tool.

Okay,now with the following description as promised:

Grayscale (K) values of grayscale pixels for example: K:188,K:205,K:230.

R,G,B (RGB across top for color of pixels to be colorized)

R: G:
B: (Channel values of colorized pixels from top to bottom)

Example: R G B
R:255,0, 228
G:162,235,228
B:162,0,255

From top to bottom: R color equivalent of K of 188: R:255,G:162,B:162 G equivalent of K of 205: R:0,G:235,B:0 B equivalent of K of 230: R:228,G:228,B:255

Example of how to colorize the pixels in the three separate images for red, green & blue:(using Bucket Fill tool & Change Foreground Color)

For example in the image for red along with the horizontal & vertical position numbers for the pixels:0,0: R:255 with green & blue in Change Foreground Color set to 0 for example.(255 meaning channel value with 0,0 meaning horizontal & vertical positions of pixel for example) 0,1:R :0. 0,2:R:228. In image for green: 0,0:G:162. 0,1:G:235. 0,2:G:228.
In image for blue:0,0:B:162. 0,1:B:228. 0,2:B:255.

Two ways to preview the resulting colorized image: Copy & paste blue & green images into red image as layers, set background (red) image as Normal,& green & blue images as Screen in Layers dialog window.

Second method of previewing resulting colorized image:(Decompose to RGB or CMY
Decompose all three red,green & blue images into layers or each mentioned red,green & blue or cyan,magenta & yellow image to three separate images. Select in Compose to RGB or CMY three out of the nine separate images or layers as follows for example:Example using Decompose to RGB & Compose to RGB:red-6/red-14, red-7/green-19,red-8/blue-24.

Before doing the above steps,use the image I included in my post to use as a tool as follows:Download & open the image that I described & posted above & make two duplicates,close original & convert second duplicate to grayscale then copy & paste grayscale converted second duplicate as layer into first duplicate. Do not merge the layers.

Leave the grayscale converted second duplicate of the tool image open in case you lose what you copied & pasted or if you make a mistake in the grayscale layer,select Undo in the grayscale layer. Zoom the image to 800 or 1600 percent. Move the slider on the right all the way up. Now choose the grayscale pixel of the image you want to colorize. Set threshold in Select by Color tool to 0.0. Use Composite in Select by Color & select in Layers window
the grayscale layer of the tool image. Click Color Picker tool twice & uncheck sample average & sample merged & check Use info window.
Click the grayscale pixel you want to colorize with the Color Picker tool & the grayscale value should appear in the info window. Using the grayscale value that appears for example 188,select the grayscale layer of the tool image. At the top horizontal row at 0,use the bottom slider to move the image left or right until the color picker tool displays in the info window the grayscale value (example 188). For example,the grayscale value of 188 will be at the position of x:188,y:0 as displayed in the info window. Click this area with the Select by Color tool. If using Gimp 2.10.2 increase the threshold any where up to 1.0. (I typed in 0.1 in the threshold slider.) Next,turn off the eye icon in the grayscale layer in the tool image & select bottom (background) layer.
Move the bottom image slider left or right until the selected area appears. For example,I moved the slider to the left until the selected area at 93,3 for red appeared,meaning the horizontal & vertical positions of that area.
Click on that area with the Color Picker tool. The RGB channel values should appear in the info window.

Use what I described above for the rest of the colorizing procedure. Any helpful replies or comments are appreciated.

reply partially edited as follows:0,0:R:255.1,0:R:0. 2,0:R:228 for red for example. For green for example-0,0:G:162.1,0:G:235.2,0:G:228. For blue for example-0,0:B:162. 1,0:B:0.2,0:B:255