Onyx
Onyx is one of Percy's favorite colors. Simply because it reminds him of Nico. Nico whose favorite gem to work with is black onyx, a small reminder of where he comes from, his father's domain. A permanent reminder of death that eventually claims everyone. But to Percy, onyx is so much more than that. Onyx is the color Nico's eyes turn when he's passionate about something or angry. It's the color of the gem in the ring Nico made for him as well. Onyx is his favorite color for all the ways it reminds him of his love, and to Percy, that's all that he needs.