I'm trying to edit the color balance of a layer in a .xcf image, and none of the color commands do anything. For example, I can crank saturation and value to 0 in Hue-Saturation, or invert values, and the layer is unaltered (both in the preview and also after I hit OK). In fact, the the log of recent actions doesn't show me doing anything when I use a color command. I checked that the image is in RGB mode and tried different layers - nothing. I have changed the color balance in layers in this same image before. Anyone know why this should be?
I have been having this problem too in GIMP 2.10.18, and finally figured out what's happening. I had the layer visible and highlighted, but the curves are adjusting the RGB channels. That means that even if you have the channels visible, you won't see an effect from the curves until you actually highlight the channels you are working on (make sure they are gray). Once I highlighted the three color channels, everything worked like a charm.
Two reasons come to my mind:
Your selected layer is not the one that you want to change. (That would be the typical error if I was you. I always get confused by what layer or mask I am currently working on)
An additional layer, higher on the stack of layers, with 100% opacity will simply hide the changes that you to to any of the layers beneath.