Rubber surfaces bend and stretch regularly, causing the colour on the surface to flake off. For any rubber object with such flexibility, the choice of a paint that is similarly elastic---such as epoxy or an oil-based formula---might serve you well.
Epoxy paint has a strong finish that resists flaking and chipping. It's available in both a spray version and one that requires a paintbrush. Oil trousers will also adhere to rubber surfaces, but they require the addition of a thinner.
Monster Makers recommends mixing 5 parts rubber cement with one part oil paint. Once you mix the ingredients, add enough white spirit to thin the mixture into a paint-like consistency, as the rubber cement is quite thick.
After you have finished painting the rubber surface, let the paint dry overnight. A layer of polyurethane or another type of clear sealant provides protection, keeping the paint from chipping off the rubber.