Similarly to Shalom's answer, I've heard that a ring used for Kiddushin should ideally be made of yellow gold, so that it is clear what is being given and the value can be fairly easily ascertained. According to this explanation, even platinum or white gold should be avoided, as they resemble silver. Having said that, there is no real Halachah, that I'm aware of, that it needs to be done this way, as long as it is certainly worth more than a Perutah.
The only caveat to what I've said above, also mentioned by Shalom and sourced to Tosafoth in a comment by Moses, is that if the bride believes she is being given something other than what she is receiving, it could possibly be a מקח טעות, or a fraudulent/erroneous transaction, which could conceivably void the entire Kiddushin. But, again, this is a safeguard, not a requirement, and as such it has developed into a Minhag in some circles (certainly Tosafoth imply that it was a Minhag at least in their locale/time, and possibly much more widespread than that, to at least avoid anything with jewels on it in case she thinks they are more valuable than they are; although Tos seem to be discussing quality of the gems, not real vs. fake - and it is possible, though I don't know for certain, that this much is codified as Halachah today).