Should nitric acid discolor 10k yellow gold?
Very dilute acid might not, but anything concentrated enough for
metal testing will leave a dark stain on 10K, and usually on 14K as
well, though it forms more slowly on 14K. The stain may even get
some greenish tinges (to the acid drop, if not the metal) on 10K.
Yellow gold seems to react a bit more than white, though this varies
a lot from piece to piece.