I have a black arrow png icon/image (with transparent background) which I'm trying to paint another color (which I sample from another image). However the bucket fill tool will only fill with black, even though I'm sure I have the desired color in both FG and BG. Is the transparent background somehow messing with the behavior of the bucket fill tool? It works as desired on another png image on a different tab.
Likely the PNG file is in Indexed Color Mode. Check in Menu:
Image > Mode whether it is set to
Indexed.... Change it to RGB and you can use any color you'd like.