There is no issue in a white gold wedding bad provided that
- it is her fiance'sthe groom's own possession
- it is worth at least a pruta (a few cents today) and
- that shethe bride is clear on what it is (see SA EH 31:1).
The custom I know is to have it pure gold and totally unadorned but chabad.org mentions that, in certain circles, it is customary to use a silver ring.
See here under 6 for further reading and here if you are interested in the halachic arguments when the ring is not what it was thought to be - it is interesting but not relevant to your question in practice.