I could not find a definitive answer with a search, I 'd like to know if colour options when they get upgraded with new paint options will also get a numerical input for RGB and Hex? For those times you want a very specific colour like for instance my crew colour I use in GTA-V.
As far as I know, the current RGB colour palette is all that is planned, as it would bring in an unnecessary amount of complexity into the gameplay for such a small feature. It could always be modded into the game in the future though.
While not the comfortable, in-game solution you are suggesting, you can modify the color presets to the exact ones you want by going to Documents\My Games\Automation and editing the ModelColourPresets.lua in which you find all preset colors you can modify to your liking.
Another question regarding the same topic: Will you add more slots for preset colors? Perhaps with smaller squares for each slot, or a vertical scroll bar?
I think going to 4x4 would make sense, the fields would still be large enough even in 720p. I too think the selection is a bit limited right now.