It's possible to paint on rubber, but if it's not done correctly, the paint will eventually chip off and peel away. The paint you use depends on your painting project.
Removable Rubber Coating
If you want the option of removing the paint from the rubber after a period of time, use a removable rubber coating. This type of paint, which you can find in spray form, works well for projects in which you aren't sure if you want the paint to be permanent, yet it is still highly durable and will not peel away on its own.
For indoor items made of rubber that will not get a lot of wear and tear, use acrylic craft paint. You may need to use two coats, and be sure to let the paint dry thoroughly between coats. After the paint is completely dry, seal it. Find a paint sealer in the paint department at a craft or home improvement store.
If you plan to paint an item that will remain outside, use a durable, non-toxic outdoor paint. Before starting the painting project, coat the rubber with an exterior primer to help the paint adhere, and let it dry completely. You can either brush on the paint or use spray paint.
Commercial-Grade Marine Paint
For maximum durability, use a commercial-grade marine paint, the kind of paint used for boats and swimming pools. This type of paint has the highest resistance against fading, cracking and chipping since it's made to withstand harsh elements.