This awesome little tool created by DreamRunner lets you see how many colours your peice has and sorts them into a nice little PNG file.
http://2ddr.com/index.php?prime=sortpal Clicking on this link will take you to a page where you enter the URL for your image and then simply click sort and then just wait for the sorting to be sorted. Currently, it seems as though it only works correctly for .gif images.
But yeah, this does have the cool effect of sorting specific hues into separate lines.