AIUI, white LEDs are a cluster of different-coloured LEDs in a single package. So your idea of colour-temperature adjustment, or similar would work equally well on (suitably wired) white LEDs as discrete colours. And if you're a semiconductor manufacturer, you're probably in a position to produce white LEDs with per-colour controls.

It would certainly simplify assembly if all the component LEDs are identical.

I always do a double take when I see LED traffic lights - the instant light is noticeable compared with the several centiseconds incandescents take. I wouldn't have believed I'd ever notice the warm-up time of a traffic light bulb, but you can indeed tell the difference!

[Edit: changed "fraction of a second" to "several centiseconds" for clarity]