All gold is yellow, its how it's mined. In the manufacturing process other chemicals are added to it to change its natural colour to red or white.
Copper is added to make rose gold, palladium and silver to make white. A 9ct white gold ring is still 37.5% pure gold (18ct is 75% pure gold) Its real name is actually "straw white".
When held beside a yellow gold ring it looks white but held beside a platinum or palladium ring, it will look yellow.
Jewellery is usually rhodium plated (a platinum colouring) to make your ring brilliantly white for your special day but unfortunately, this will wear off over time.
Depending on your job, or whether you use cleaning chemicals or abrasives, this coating will wear off quicker allowing the natural colour to come through.
As rhodium plating is a "pure white", your originally wedding ring will look yellow. I can assure you that the ring is a white gold which any jeweller will confirm.
There are 2 options open to you.
1. The first is to have your ring repolished & rhodium plated
2. or secondly, have the rings polished back to their natural state. The removal of the original plating will allow the beauty of the warm white colour to shine through.
The bottom line is that there is no such thing as white gold... By law the 9ct alloy must contain a minimum of 37.5% gold with the balance being other metals, which for white gold, attempt to bleach out the gold colour.
With better quality white gold the inclusion of germanium helps this process no end, but if it is left to zinc to do this process the result is that you are left with an alloy that can react with sulfides and oxygen to create a pale yellow patina..
So you cannot stop this process except to have the rings either rhodium plated or palladium plated.
Rhodium being the better choice.
(Osmium would be better but the cost is prohibitive).