Don't use a grit finer than 800. I made the mistake of taking 3 days to sand mine down to 2000grit. It's difficult for paint to stick to this smooth of a surface and resist chipping. 800grit is the number a friend/pro painter gave me.
Yes, my wheels were graphite (duplicolor), then midnight black metallic (rustoleum), then cobalt blue metallic (rustoleum), and now florescent green (rustoleum). The 1st time I painted the graphite, I aircraft stripped, sanded, cleaned, painted, and cleared. After that, I got really lazy and just used a scowering pad rub off loose stuff, cleaned with denatured alcohol to prep. When I did the green, since it was a dark undercoating, I aircraft stripped, white primered, then put on the green, and then the clear.
There's a lot of ways to do it, I don't really have a recommendation over stripping the old paint and sanding vs light sanding, clean and paint over. Reason is because both have their advantages. Advantage by stripping is that you start fresh. PITA because it's hard to get every little bit off w/o being OCD about it, and little things turn out important during prep.
Advantave from painting over existing paint is that you know that it already has a good bond to the paint. If you rough sand it and clean over it, it's easier to get a decent finish with less work. But then you have the color underneath affecting the final color when it dries and etc.
There are a lot of tips and trips that I could probably make an entire how-to about, but so many exist out there now it seems redundant.
"We live for a good time, not a long time."
RIP Colin McRae