I'm not sure about machining an aluminum trackball. I think balls are very hard objects for a person to machine by hand. However, I bet you could find a big chrome bearing somewhere. I've seen them a couple of inches in diameter before. The hard part would be finding someone who knows where to get one!
Don't paint a ball, that won't work for sure.
The dye will probably work, I've dyed plastics before and it works very well. I suspect it will work on a trackball too.
I recently bleached my white ebay trackball and it looked 10 times better. I still wanted a black one for my cabinet though so I bought one from Bob Roberts. I have to say, it looks REALLY nice! It also rolls smoother and faster than my old one too.
I totally forgot about being able to dye plastics. This would be a great way to get custom color setups for buttons. You could also dye your Start buttons a different color than white! Very cool.
Wade