Actually you can´t do that, look at this thread http://www.wc3campaigns.net/showthread.php?t=85410. The only way of making custom team color is by modifying the mpq file.....
There is already white dude...
EDIT: looks like im wrong, its gray...
hold on, ill type up how add/replace a certain teamcolours..colour.
Just extract or create a single coloured image and convert it back to blp.
and change its path to either of the following when you import:
ReplaceableTextures\TeamColor\TeamColor00.blp
ReplaceableTextures\TeamColor\TeamColor01.blp
ReplaceableTextures\TeamColor\TeamColor02.blp
etc..
each number is a colour.
eg.
red=00
blue=01
etc