From left to right: original image, after applying Color to Alpha
filter (Blue to transparency)
You can find this filter through
-> -> .
The Color to Alpha filter makes transparent all pixels with a
selected color. An Alpha channel is created. It will attempt to
preserve anti-aliasing information by using a partially
intelligent routine that replaces weak color information with
weak alpha information. In this way, areas that contain an
element of the selected color will maintain a blended appearance
with their surrounding pixels.