@DUTY OF RS
fail
I think that there is a little image (1x1 probably) with a certain color making the tooltip an RGBA color from that .tga or .blp.
(Same how backgrounds of multiboards work.)
However, I dont really know what that file is called, you should try searching an MPQ editor.
EDIT:
You can change the background color of tooltips by going to Gameplay Interface and edit the value "Image - Tooltip Background".
This only has to be a 1x1 blp (or tga dunno) image so it has a good color.