I had a problem that has cropped up occasionally before but I never bothered investigating. I needed to make a multi coloured logo into an all white and an all black logo to be printed on dark and light shirts respectively (screened or DTG in white ink and black ink).
So I converted the logo into a white PNG with a transparent background. The quickest way to do this is to use:
Image > Mode> Indexed. That should convert the whole thing to black, then to get my white logo I just use Color> Invert.
But of course, I had to show what it would look like on a coloured background.
When I used the GIMP bucket Fill tool, and selected a nice blue background colour, the fill kept putting the default black BG colour into the transparent area. Swapping to FG just filled the transparent area with the default white. Nothing would make the tool fill with the blue I chose.
Then I remembered. I was working in 'Indexed Mode' which is how I converted the coloured logo to pure white in the first place!
The fix was simple:
Image> Mode> RGB
Then the Bucket Fill tool allwed me to fill the transparent areas with Blue.