Gimp color tools (colorize, brightness, curves, etc) aren't working
Solution 1:
Ah ha, I just noticed this itty bitty error message at the bottom of the screen:
I forgot my image is in indexed color mode, not RGB! My PNG used to be RGB, but compression software changed it to indexed. To get all the color tools working again, just convert it back to RGB:
Now color tools work, hooray!