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.
I had the same issue. A black and white image - actually black and transparent. I want to turn the black to white. Colors > Invert does nothing. Selecting by color (black) and using Bucket Fill set to "Fill whole selection" using the white background fill does nothing. Edit > Fill Background Color does nothing. I have the layer selected and visible, and I have both Channels selected (Indexed and Alpha). Also tried with only the Indexed channel selected.
The solution: This doesn't work with Indexed mode. I change the mode (Image > Mode) to Grayscale. Success!
In order to apply color changes, it is not enough just by selecting the layer. You have to actually select the element you want to change. If you want to change the whole picture, click Ctrl+A, and then open the color options. You will be able to see your changes get applied then.
I was working on a very large format multi-layer image and I couldn't change the color of any layer. I had inadvertently selected a tiny area on a lower layer. It was so small I didn't notice the "ants marching" selection marking. Every time I tried to change the color attributes, it was only applying the change to the tiny unnoticeable selection. I finally hit "select - none" and bang I was back in business!
I was having a similar problem and I found this question here on the Web. I'm not exactly sure if this is the problem you were having, but I found this helpful. I opened a .png image with GIMP 2.10.30 and tried to edit the foreground and background colors, to no avail. Really, for my purposes I only needed the canvas size. So, I started a new project from scratch and then everything worked fine.
I hope this helps!