You can use the keys to claim a car. You move away from it to build something, then move on a bit because you've seen something doesn't work right just some steps away (so going back to the car and driving there would be a waste of time), then something else pops up still a bit further … Finally, you want to drive to an outpost or your power station. Instead of walking all the way back to the car, you can just summon it near you and get in, then use Autodrive to do the actual driving to that far-away outpost. I use this a lot in my own game. :-)
The coloring probably is only useful in multiplayer, so you can identify "your" car even if some other player has used it while you were away. Locking a car also doesn't make much sense in single player.