Ok,
mycellium is only a third correct on his evaluation of your question.
In the industry I work in, the grip department has all the reflective elements to help cast certain details to the image.
gold, for instance, gives a warmer look.
silver gives a shinyer, hotter, look.
gold/silver checkerboard a warm and hot look.
white gives a softer look.
black gives a negative look.(you don't bounce light with black, but you cut the light in certain areas of the image to give less light for a negative fill.)
when ever you bounce light however, you tend to lose at least a stop depending on what your light source is.
as far as your umbrella rig is concerned, the umbrella has a natural concaveness to it, that is why it a popular item for photogs. The light that is bounced back from the umbrella will direct the light to a focal point better than say a flat peice of white foam core.
The warm light will hit your subject where you want.
A yellow, or any filter, will cover the whole image and not just the subject itself.
good luck.
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