Like Lightor mentioned, both Lightor and I were tired of not having this option in game.
We teamed up with Brainslug and Lugburz and have a userscript (lugburz' much used racing ui enhacements that can enable it.)
Currently I photoshopped all class a cars in 12 different colours (fresh images for most too).
These can then be assigned by me towards people for everyone who uses the script to see.
I am the organisor of Wreckfest Knockout Tournament all the winners of the tournament get a special edition car. Which I know many othertournament organisors want to supply too.
As for the coloured versions they are basicly into existance because Lightor and I are working on a new tournament which will use them.
As for how the other people can acquire them we currently still have to see exactly how to, but we are first doing a small testrun to get the last kinks out of the way.
Its our way of showing to the devs and ched seriously guys this had to be an option already
Racing Car Skins UPDATE?!?!? Maybe