Go to the hardware store or any Lowe's or similiar store and look for the Krylon Fusion spray paint that is for plastic applications.
If your wheels have a finish on them, you will need to remove the finish.
If they are a lighter color then black and you are wanting them to be black or a darker color then what they already are, and they do not have a finish on them, You can dye them instead of painting them.
The dye will hold up alot better then the paint will.
And also, you will get more help with this subject if you could post it in the correct section.