GIMP odd behaviour Some parts don't work

[Ofnuts]

Yes, as @denzjos said, you are probably editing a color-indexed image (typically if you are editing a GIF, but some PNG are also color-indexed), and given how these images are made, most color tools (and many others) are disabled. See here for some hints about color-indexed images and their pitfalls.

In the general case, uninstalling/reinstalling Gimp doesn't help, because many of your problems will come from settings you changed unknowingly, and Gimp keeps your settings across installations.  See here for some hints about managing that and making Gimp forget your errors