Personally, I can't stand it when people mix yellow gold and white gold together.
I don't see how it matters. Your ring is on your finger, the rest of your jewellery is up on your head (unless you wear a bracelet too), they're not sitting right next to each other.
Having said that, my e-ring is 2 tone anyway, so I could have got a yellow gold or platinum ring to match it, but I chose yellow gold.
Don't stress me out people! *runs around pulling her hair out* I never thought of this, but FH has bought me a beautiful sterling silver and pearl necklace as my wedding present to wear on the day, and I WILL wear it with my yellow gold ring! hehe