I think you should not invert the colors, but the selection.
I mean, use the fuzzy select tool, select the outside of the arrow, then reverse the selection by going selection->invert
THEN filling it with you foreground color, either by edit->fill with FG color, or even the bucket tool
Something like that?
But really, maybe Rich has a better approach. For something as simple as an arrow, maybe even drawing it with the pencil or paintbrush tool using straight lines with shift or shift+control for certain angles, and then just bucket filling it.
By the way, I like the artwork there
I mean, use the fuzzy select tool, select the outside of the arrow, then reverse the selection by going selection->invert
THEN filling it with you foreground color, either by edit->fill with FG color, or even the bucket tool
Something like that?
But really, maybe Rich has a better approach. For something as simple as an arrow, maybe even drawing it with the pencil or paintbrush tool using straight lines with shift or shift+control for certain angles, and then just bucket filling it.
By the way, I like the artwork there