I was trying to submit an app to the Windows Phone app store. Here all pixels need to be white (#FFFFFF) or transparant (#FF####).
I had a black logo on a white background. To make the background transparant and the foreground white I used Gimp. First I made the background transparant using the magic wand and changing color to alpha. Then I inverted the remaining colors. Unfortunately the images did not get accepted in the Windows Phone app store because not all pixels were white or transparant.
How can non-white or transparant pixels in Gimp be converted to white?
Are you sure you need the GIF format? PNG would allow smooth edges with transparency.
To edit the output colors you should first set your color mode to Indexed (
Image > Mode > Indexed). For the conversion select
Use black and white (1-bit) palette
Then you can edit the color table (
Window > Dockable Dialogues > Colormap). Just doubleclick the black color and set it to white.
Edit: For a PNG you can use Colorize (
Colors > Colorize) where you can set
+100 to make everything white. This will keep transparency intact.