I see and hear of this occasionally, it even happened to me once when we cleaned a 1.00 ct F color Princess cut with the last few rusty bursts of dirty steam from an aging steam cleaner.
The F color diamond suddenly looked like a J/K color with no obvious dirt or minerals showing on the diamond.
My theory is that the iron (rust) has an affinity for the diamond and sticks to the surface and/or in the hollowed out area of the prongs that are designed to protect the corner of the princess cut diamond. With even a trace of dirt or minerals contacting the princess cut diamond, the diamond will look noticeably tinted.
Here is what I do to remedy this problem, I use alternating baths in a pickle solution (heated acid bath) and heated ultrasonic along with a steam cleaner to help steam out any particles etc. This process takes patience, as it takes much more time and repeated cycles of the process than an ordinary cleaning.
As a home solution, you could maybe try some more soaking in vinegar or even a CLR (calcium,lime rust) type cleaner as long as it does not have any chlorine in it (bad for gold)