Either use Krylon Fusion spray paint, which is designed to bond to plastics, or use vinyl dye. Vinyl dye can be found at auto parts stores, and I guess it's intended for re-coloring trim pieces on your car or whatever. Anyway, it's a mixture of a dye and a solvent, so when you spray it on it soaks in, and once the solvent evaporates, the dye is left behind. Unlike paint it can't chip off, 'cause it's not a coating, it has actually penetrated the plastic.
Just a caution, the solvents used in vinyl dye are very toxic. Make sure whatever you paint with it has thoroughly cured before you put it in your tank. Spray outdoors and let it dry overnight, then bring it inside where it's warmer and keep it somewhere that's well ventilated for maybe a week or so. Vinyl dye is nontoxic once cured though. Reefers have reef crust growing on PVC they've dyed, and reefs are generally considered super-sensitive.