I'd like to create colorful crystal ball images identical to these, but using different colors:
http://www.psdgraphics.com/graphics/colorful-3d-crystal-balls/ (also got one attached here)

I spent a few hours wrestling with Colors > Map > Color Exchange, but not having any luck. I've tried sampling a number of different locations as the "from color", and many different values in the "to color". Then with "lock threshold" checked I drag the thresholds up until everything in the image is selected to exchange. The results are consistently pretty ugly, not the nice even gradients in the source files with plausible shades, reflections, etc.. They look white-ed out, splotchy, ...a little hard to describe.

Any suggestions on how I might better manage a smooth exchange to new color neighborhoods?

You could try:

Desaturate the original image (using luminosity)

Add a coloured layer on top

Change the layer mode to overlay

rich: www.gimp-forum.net

On 12/18/2017 10:38 AM, ottenm wrote:

rich404! Can't thank you enough!

Here's the steps I'm following:

Colors > Desaturate > Luminosity Layer > New Layer > Transparency
Select foreground color > bucket fill Window > Dockable Dialogs > Layers > adjust opacity and mode for this layer (both at the top) to:
Overlay, Screen, Addition, Hard light, Grain merge

Great results overall. Struggling to create some brighter spheres as the desaturated originall image is very dark/black (so even if I foreground a bright color, it darkens significantly as I bring in the other layer). Have tried tweaking the brightness and contrast on the desaturated original, but the reflection spots blur and loose definition.

Still, had none of that in my original question and your answer really helped. Would never have found it on my own. Thank you!

Here's another function to check out: Open your original image and switch on the Hue/Saturation tool. Click on the image and in the toolbox that opens, give the Hue slider a try. It seems to do a very good job here, with the first sample image you provided.

:o)