Show a Forbidden sign with Paint tools and show/raise the colormap dialog if someone wants to use a color that's not in an indexed image's colormap
Operating System: All
Description of the feature
Users wondering why they can't use a color to paint - or why they get a different color instead - is a frequent occurrence with images using a colormap or "indexed images". These days, this is often textures used in Minecraft or similar games, where a limited number of colors is part of the games visual identity.
Showing a Forbidden sign next to the tool cursor and highlighting the colormap dialog (similar to how the selection tool cursor is shown, and the tool mode is highlighted when a mode is impossible to use) might make this a bit more discoverable.
This focuses on keeping the images in indexed mode, which may or may not be the correct one. What has to be considered is whether having users change the image mode to RGB is the more desirable approach.