I have an image where i want to allow someone to pick which colors are of custom alpha level. So for example, they could click on pixel (x=5, y=23) which, let's say is Color(245,123,119). Then they say, that color is alpha 0.4. So everywhere in the image where that pixel appears, the graphics object will paint it with an alpha level of .4.
You might be able to use RGBImageFilter ( here's an example of using RGBImageFilter to replace one color in an image with another color) not sure if this would work with alpha values, but it might if you specifiy alpha values on the original color.