You can have a custom Crew Color in GTA Online through the Social Club website, but it only allows you to choose from a fixed RGB palette.

To be able to paint a car pearlescent in GTA Online, players need to have a base metallic or crew color in place. Pearlescent colors make the cars look like they have two colors on it at the same ...

Discerning GTA 5 tipster, Granty, reveals how to get secret crew colours on your car and easiest ghost glitch in GTA Online.