Why don't custom maps let you choose your own team color?

[Toppers]

Just idle wondering. Are there team-color reliant triggers or something? I figure in something like DotA it's just so you don't get confused about which colors are on your team or something like that, and so you can immediately recognize that that Omniknight IS in fact an enemy and about to kill you.
But what about on something like Sunken City that, for the most part uses assets that don't really look at all similar to any of the heroes?

 

[GenoHacker]

Most maps that use custom forces most likely also have Fixed Player Settings

 

[Elektrohund]

iirc player color is tied to player number, which means that player 1 will always be red and player 2 blue when referring to them in triggers, unless defined otherwise. So if a map has player 9 (grey) act as the hostile AI, selecting that color in the lobby would cause issues.

 

[Tasyen]

One can toogle enemy/friend Color / normal color usage with one of the buttons right to the minimap.

 

[Wark]

Fixed Player Settings

It is possible to design your map to not use "fixed player settings" if you want to choose your color.
I'm doing it on one of my projects with computer controlled factions.

What Tasyen mentioned is the DotA feature, where allies and enemies are a set color, and you can even fix it to be like that always.
For assets on Sunken City, what do you mean?