I made this for my own reference so I wouldn't forget how to do this, but I put it up here in case any other Painter users might like to see how I paint magic balls. Took a lot of messing around to figure out how to make it look like water instead of blue fire, but I think I managed. It's ultimately for my brother, and he's pleased so far; that's all I need.
Made with and in mind of: Corel Painter IX, though it'll probably be the same in older and newer versions of Painter, and with some creative free thought, bits could apply to Photoshop.
EDIT: If you don't plan on having a white background, then you'll probably want to turn on"Preserve Transparency" for the layer with the magic ball when you get to step 5. I haven't tried that yet, but given what I understand about Painter, it'll work.
EDIT2: Never mind. Finish the pic COMPLETELY, do steps 1-4, then click "Pick up underlying color" and proceed with 5 and/or 6 as desired.