In somebody's cases, the gold jwels become red in color, thought the purity of gold is good/standard. why?

Answer 1

This is wrong information.
In the gold jwels mixing with copper. So Gold Jwels become red in colour.

Answer 2

Gold is pure in nature. But with pure gold no jewellery can be made, So we need to add some impurity into it. That red/black color is due to the impurity present in the gold.

Answer 3

I've heard that all kinds of jewelery tend to get rusty or red or black when you put too many together in a small place.
So get them cleaned at a jewelery store and then buy them separate boxes!!!
If of course that's the reason!