Judy, I'm sure there is paint available that would stick to polyethylene--at least for a while. Have you condsidered though, that painting the boat would make it heavier? Maybe not much heavier, but it is my experience that nearly every pound you add to a kayak is significant. If you are serious about changing the color, you might want to see if there is such a thing as colored water-based urethane. I have used clear water-based urethane to coat a poly boat and so far it has not peeled off. Preparation is key.
I can think of several reasons why black would not be my first choice for a color for a kayak, but I've seen some that were quite attractive.