I'm currently busy editing one of my campaigns, but once I'm done with that, I'll spend quite some time with this and try to accomplish what we are working on. I don't know, but I feel like Blizz had a reason to do that - perhaps they wanted to patch in more colors later? Or sell them with an expansion and never did that? It doesn't matter, we'll keep trying - even if I have to edit the whole mpq for that.
Neutral Extra and Neutral Victim can be used with triggers. I don't know for certain about extra, but all units that belong to neutral Victim are attacked by player units and don't defend themselves.
You can use the colors with Player - Set color to color of player Neutral Victim/Extra. So yeah, two more colors that can be activated via trigger.
Edit: There is also one more issue about custom colors: The scorescreen.
Neutral Extra and Neutral Victim can be used with triggers. I don't know for certain about extra, but all units that belong to neutral Victim are attacked by player units and don't defend themselves.
You can use the colors with Player - Set color to color of player Neutral Victim/Extra. So yeah, two more colors that can be activated via trigger.
Edit: There is also one more issue about custom colors: The scorescreen.