if it's plastic (polyethylene or PE), I'm a afraid you can forget it. Either live with the colour or get another boat. If the boat is really shiny you may try to use something called \"fablon\" (used by lettering companies) to cover the top but it probably won't last very long. At least it will get damaged. Don't cover the bottom of the boat, before you've paddled away from the river banks it will be gone.
If it's polyester (or at least has an outside layer of gelcoat you could use something called mono-coat or double-coat (respectively a one-component and a two component type of paint). The mono coat variety can be bought at a paint store (your average DIY shop probably won't have it in store though, it is used for painting sail boats etc.). But remember, this stuff won't stick to plastic
Good luck