... Timber ... , Mar 19, 2004; 01:12 p.m.
Black does not work as a bounce card. Black could be used to temporarily reduce the effective reflection area of a white card only.
As for black reducing any tendency of "washing out" your photographs. I am puzzled. But I am believing that you are an "auto mode" photographer (GAMP) and you are manipulating the sensor input of your auto mode flash by using a black card in some way.
White plastic is more long lived than white cards from paper. So, I use white plastic. The larger the white card, the more total power output I gain from it. So, you can experiment with size. Mine are much larger than average, about 8x10 or 11x14 with rounded edges. I have smaller ones, or I can reduce the size of the white card by using pin-in black plastic. This is all custom stuff. TAP Plastics has the white plastic material that will bend with 150 degrees of heat, and then set. It can be re-heated and re-set many times, too.
A yellow bounce card is interesting, but i wonder why I would ever use it. A white card loses 2 f stops of power, and yellow should be more. As a result, the yellow card effect would only be seen at close distances, like 5 feet, not 11 feet when the reflector is used for bouncing at the ceiling.
Perhaps you are using the yellow card as a yellow filter and not pointing the original silver reflector at anything but upward. In any case, a yellow filter over the reflector can do this trick more efficiently.
Using a black card as a snoot or partial barn door is an idea I use. It allows the photographer to use a reflector with white card to be used as a barn door: just attach the black card with velcro to the white card and you have a barn door! (one barn door, that is)