I couple of important buttons I think you guys should have: BLEND and NO BLEND, and REPLACE.
(No) Blend is a simple toggle which dictates whatto do with an alpha color. When on, your color blends into the background. (RGBA(128,128,128,128) might fade to a light pink while on a deep red background, for example.). When off, the area selected becomes that value, instead.
Replace simply lets you choose a color in the image, and then replace it with another. You could select black, for instance, and with a single click turn it all red.
In combination, the two provide the extremelt useful functionality of letting one choose a 255 alpha black, and transform it into a 255 alpha clear, before saving.