Have you see the web sites with many graphics? they have solved that problem easyly.
Think about a puzzle, divide the image on parts than can be changed, when the user wants other color, just put the other color piece of the image and thats all.
Only you have to manage the files than would be downloaded. Other aproach is to have a java applet to do the task and you don't have to have a dll.
Mauricio
Be fast like the Fox
Be free like the Penguin
Se Rápido como el Zorro
Se libre como el Pingüino