By default, it's solid black, or 000000. The color 000001, the closest to black, will not be transparent, except if 16-bit color is used (use 000008 instead, the closest you can get with 16-bit color). Instead of black, there's a predefined variable that gets the transparent color from the top left corner, bu I don't know what it is. If you want partially transparent areas, use an image with alpha channel and save as TGA.
