Aspect warriors have set colors. Granted, these colors can change, but the basic colors used stay the same.
Striking Scorpions are green, generally with yellow
Dire Avengers are blue, generally with white,
Dark Reapers are black, usually with purple,
Howling banshees are less strict in color, but generally a bone color.

Warlocks won't have a problem with this, but how would one signify oneself as being part of a specific craftworld if you were an aspect warrior that had a recognised, consistent color?
DISCUSS!

Well my table top eldar are exodites here is the color scheme of MOST aspect warriors

Well... if you are from Biel-Tan, you can simply paint a little <3 in your helmet and you are good!

Is it only DoW or is the color of the waystones not different too, depending on where you come from?

Don't really know... there is no official colour, so I guess they could look different. I think that they usually turn glowing when they inhabit someone though.

So are they painted in TT or what? (obviously they are.... anyway: how does one decide which color?)

like i said, howling banshees have more leeway than any other aspect warrior.
they can be any color.

Let not the colors of our banners detach us my kinsman! No matter our uniform, we all fight to bring the sunlight of Isha to Arkhona! Should it be wished, I'm certain an artisan would be willing to affix the rune of ones Craft World to their armor. Yet I see it as inessential. When our descendants and far away kinsman learn of our victory, they will hear how each Craft World was a vital tool for the task.

The aspect colors are set yes but very depending which Shrine the aspect warrior comes from, no numbers have been given to how many shrines there are or if the same shrine can exist on multiple craftworlds to my knowledge.

Black is the new black, suckas!