Figure 11.47.  From left to right: original image, after applying Color to Alpha filter (Blue to transparency)

You can find this filter through Filters->Colors->Color to Alpha.

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.