Brush painting with background color instead of foreground color

[Ofnuts]

What are the "brush dynamics" set to? Some brush dynamics operate across a gradient, and if your gradient is the usual FG to BG  and the brush uses the end of the gradient you would get this behavior. Colors from gradient and a short fade length woul dbehave that way.

No need to reinstall Gimp to reset settings. See here for hints about managing your Gimp options.