Because Percy could manipulate water, he could form rainbows whenever he wanted. It took him a while, but eventually he mastered the technique of bending water precisely where he wanted the rainbow to appear. This all depended on the amount to light present; On cloudy days, it was more difficult to do, but Percy liked the extra challenge. Whenever he was happy, you could tell because a giant rainbow would form above him. When he was bored he would sit and play with the different hues of light, and when his baby sister was learning colors he was her favorite teacher. He had the most fun, though, after Will and Nico got together. Every time he saw them doing anything remotely cutesy together, even simply bickering, a rainbow would appear around them.