I know about the transparency threshold in the GIF save dialog. What I'm saying is that pixels above that value but not fully opaque shouldn't be made simply white. Instead, their transparency should be bumped up to 255, thus preserving the color.
For example: if a pixel has color RGBa(1, 2, 3, 200) it should be made into RGB(1, 2, 3), instead of RGB(255, 255, 255).
This would be incredibly useful in a number of cases, and I see no situation in which such a behavior would be unwanted. If you consider there are such situations, a checkbox in the GIF save dialog would be nice.