It sounds like these are separate layers and they all need to be colorized individually. However, from what I read you are doing this. Any chance of sharing the file? I also wonder if when the image is opened in GIMP if exporting it as a .xcf might help GIMP correctly preserve your work. After all .psd is a photoshop native file and while GIMP will open it there surely are features that are not compatible between Photoshop and GIMP. I wish I could be more help, but without seeing the file itself it is hard.