How to fill alpha channel in GIMP?
I have an image with an alpha channel that I'd just like to fill in with one color (e.g., for a sky). I try using the Magic Wand, but it leaves white borders around where the alpha channel and image meet.
How can I fill in the entire alpha channel with a foreground color in GIMP?
Another method, that works on the same layer: bucket fill you whole layer after setting the bucket-fill to "Behind" mode. As its name implies "Behind" only fills transparency, and "tops up" partial transparency exactly as if you added a background layer.
Create a layer and put it underneath your current image.
Then fill in the entire layer with your foreground color using the Bucket tool.