What version of the game you are experiencing the issue with:
Cata Classic

Describe the bug
In Cata Classic I have the enemy health bar set to "By Threat", and while I am tanking it shows the enemy color as "warning" color instead of "attacking another tank" color. When I am tanking it appropriately uses "safe" color.

Expected behavior
Should use "attacking another tank" color when tank has aggro.

I'm dumb working as intended. I did not have "highlight mobs on off-tanks" and "health bar color" selected. My bad.