This is not that uncommon of a question. The first rule of thumb is that if it does not look like gold, then it probably is not gold. However, there is one exception and that is when the sputtering process creates some kind of surface structure that leads to an optical effect, there may be a nongold looking gold color (more like gray). But this typically does not adversely effect the sputtering rate.

Other cases would be when either a) contamination from external sources (e.g., fnger prints) or b) build up of contaminants from the use of gold with insufficient purity. If a), then the “problem” is solved by a good solvent washing and scrubbing with something like acetone. If b), then it will not rub off with solvent and it would be something building up in the way of impurities from within the gold foil itself.