How to change all pixels in Gimp that are not one of two colors to white?

Question

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?

Answer 1

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 Lightness to +100 to make everything white. This will keep transparency intact.