I have been trying to get to grips with luminosity masks for a couple of months now - on and off. After reading the Gimp tutorial and watching a couple of You Tube vids I think/thought I understood them :-)
On a photo where I wanted to lighten the darker bits I produced a set of luminosity channels ie L, LL, LLL D, DD, DDD all went well and seemed to be working. I took the DD channel and produced a layer mask with the idea of lightening this part of the image. My problem, which has me tearing what hair I have, is that - when I apply increasing the exposure, or lighten or darken modules, from the colour tab, while it lightens it also introduces a colour cast ! And further to this if I then try to turn the photo into a Mono, the colour cast still remains although the original photo turns mono. I have thought about this and tried whatever I can think of, nothing has worked and I am at a complete loss.
Hope I have explained my problem well, I have run out of ideas and would be grateful if some knowledgeable person could point me in the right direction.