As characters are supposed to represent the 'most interesting' aspects of the meta, I believe it would be thematic for player characters to have access to not only the '@colors vehicle' option that is available currently only when buying a vehicle new, but indeed to the entire '@color 256' palette through a system similar to what is already in place.
The repaint menu would likely need to be changed from it's current iteration into a 3-step process.
Step 1: Pick the color family. This is actually already the current step 1, so not much would change here!
Step 2: Pick the specific color. When you pick a color family, you are greeted with the colors within that family that you have the skill to produce. Each of the 256-color options fairly easily falls into one of the 14 from earlier.
Step 3: Select your finish. Here, you pick the finish option, be it metallic, chatoyant, or any of the other twenty options. This list would be again, populated by the finish option you have unlocked through your skill level.
upon completing these menus, the repaint process proceeds as normal, except now you have much closer approximations of the color that is actually applied to the vehicle, and more variety for the player to pick, as well as a much more true to the flavor text depiction of what is happening (Every paint is custom mixed every time, according to the baked in poses).