Accessories

I don't see the photo of your ring, however, I think the only person that will notice that you mixed jewelry colors is you. If the tiara is gold and silver, it should still coordinate well anyway. I personally wouldn't wear a necklace and let the neckline of the dress shine, but I think something dainty and thin could also look nice.

Your dress is so gorgeous. I wouldn't wear a necklace and would either wear a really nice pair of stud earrings (either metal would be fine) or statement earrings. Since you are doing a tiara maybe keep the "metal" on the earrings to a minimum. What I mean by that is

Something like this

Instead of something like this

I absolutely mixed metals and I don't think anyone noticed, or at very least cared. My engagement ring is rose gold, my wedding band is silver, the necklace I wore was gold and my watch was gold and silver.

I think mixing metals is totally fine - if your tiara is gold and silver, that will tie everything together nicely. I'd do minimal jewelry to let your dress and tiara be the stars though.