Why Aren’t The Colours Working Properly On Gimp?

I’m in the middle of an animation, but for some reason the colours have suddenly gone all funny and are coming out the wrong colour after being selected. For example, every time I try to use red, the red colour selects well enough but when I use it with the paintbrush it comes out a light brown. I’ve tried using the pencil tool, exporting it as a gif, restarting the program…. but nothing seems to be working! Does anyone know how I can fix this?