Question re editing bitmap images in themes

[iPodVT]

I'm starting to get into theming, and as a former programmer I'm doing okay with the coding.  What's currently tripping me up is creating and editing bitmap images of controls and gauges with gradient coloring that transforms and blends into a surrounding different color.  I understand how multi-paned bitmap files and masking work.

Do graphics editing applications have standard tools/effects that can be applied to an image element in order to add or change its color grading and the blending of the element's edges into the surrounding color?  (I'm not sure if these terms I'm using are clear, correct, and/or standard.)

Thanks.