Most of my pieces and my wedding set are in yellow gold, as that works better with my skin tone and is my personal preference. However, I do have a right hand ring in WG, and I feel that different hands are far enough away from one another not to clash. I also occasionally mix necklaces, but it works with some pieces and not with others, so I think a person just has to experiment with what they have. Mixing metals can definitely work, though, so don't be afraid to try it.