That's why I always liked the paint chip system for some reasons.
And before people start to be mad at me, I will explain: with a color picker it would be really hard to get the right, realistic tones for most manufacturer colors. I think the best option would be if we had the paint chip system plus a paint creator, where you could mess with the layers in the materials.
Gran Turismo does a great job representing the correct manufacturer colors, I've not seen any game do it in such accurate way. And that's because those layers in the materials are correctly done. They've got variations in the coat layers, pigments, the "metallic flakes".
Just notice how cool and realistic are paints such as the "Redline 3 coat" (from Ford, I think), The Black Opal Metallic (from Nissan GT-R black edition), Etna black (from Alfa Romeo MiTo), Cosmic Blue (from Volvo, S60R)... see also some iridescent paints from Alfa, such as Azurro Nuvola.
There's a lot of other examples, of course.
Imagine the hell of trying to replicate the TVR colors accurately with a color picker...