RGB Red Green Blue LED tailights on cars. maybe some day?

I'm guessing they aren't quit there yet, not bright enough maybe? Plus the cost versus a couple of bulbs and some colored plastic. But I think its only a matter of time.
Instead of 3 or so different colored plastic elements, just the one element with a clear cover that can be any color , or combination of colors you want, within the law. :) All Red, adjust the brightness for brakes on of off. Or Yellow, or Orange (Red and Green) for turn signals. Or all White for backing up (Red, Green and Blue). Even Blue if its say a police car.
This kind of tech, 64x64 RGB LED Matrix - 2.5mm Pitch [1/32 Scan] ID: 3649 - $74.95 : Adafruit Industries, Unique fun DIY electronics and kits

Buses and trucks around here have LED taillights, single color LED's though like all RED. The triple LED's RGB likely aren't bright enough to pass the standard.

I think its more about reliabilty. If one or two LED's burn out its no big deal, you can still see the tailight. If the bulb burns out that whole element is dead. Lots and lots of LED daytime running lights out there now. And in unique shapes.