Go see a professsional hairdresser and colorist. Never change your hair colour more than three shades deeper or lighter than your natural hair colour. That's a good general rule that countless hairdressers have quoted to me and that they live by. Nothing looks worse than blonde streaks in dark hair and someone with a pale complexion going raven black! Let a pro guide you.
You could always experiment with those wash-in, wash-out, non-permanent colours. Clairol makes good ones and they are cheap enough and wash out enough that you can try several over the course of several months and see what sort of colours suit you. I found out very quickly that anything with red tones looks terrible on me! I'm glad I found that out before committing myself to it on a permanent basis at the hairdressers!!
Clare