I have always just painted them Black with latex paint. The bad thing about that is that if anything like a filter stand-off rests on it, it can stick to it and pull off. The good points are non-toxic, can be touched up quickly, and it comes off super easy when you want it to. I just wet it w/ hot water and it peels off in one big rubbery piece. The other thing I have done is used self-adhesive black sign lettering vinyl (I work in the trade so I always have access to it). This is the best, as it is water proof & durable, but for the layperson to go buy it it is expensive. A quart of semi-gloss latex or acrylic paint is less than $4 and will do many tanks.