I think Cheyenne may be referring to the HotSpot that brings up the circular hotbar.(But I may be wrong)
The color is set by the image file that contains that image. If you want to edit it, open the relevant file in photoshop or something and edit your heart out.
As for the fade time, I'm not sure, haven't looked at the XML for that window as I don't like it personally.
Now if you are talking about an actual HUD (The thing that displays health, energy, ect.) Then the color would be the same method, open in an editor and change as you see fit.
The HUDs are timed to disappear after your target has died, or you de-select from your target.