It depends on what you're trying to do. If you're trying to make a transparent sector, make an FOF, only change it's type to "solid, transparent" or something like that. If it's a color map, the letter at the end of the bitset thing controls transparancy. a is the most transparent, while z is least transparent. I think. Perhaps vica versa...?
Either way, please explain with a little more detail. I can't help you with objects or anything custom like that.