White hulls reduce air conditioning costs. That said, not all ships are white. Holland America Line opts for a dark blue that reflects their heritage sailing the cooler trans-Atlantic routes, though now they also go to the Caribbean, Hawaii, etc. read more
Celebrity is white with navy blue - and the greater part of its newest ship, Edge, is navy. Holland America also uses a lot of navy blue in its color scheme. Norwegian paints brightly colored murals on its ships, as does Aida (the background is white, though). read more