Scratch

The most efficient thing I can think of: Take a sample of the picture behind the sprite, only looking at pixels that are covered (beneath opaque, not transparent pixels).  Iterate through those pixels.  If you find a pixel whose color value is equal to the one you're checking for, stop searching and return true.  If you reach the end of the sample without finding a match, return false.  As long as your images aren't huge, it shouldn't eat up too much overhead.