Hey
I have a US strat which has been repainted before I got it. Is there any way to tell what the original colour was from the serial number?

You can't tell from the serial number; the serial number will only get you the approximate year of manufacture.

However, you can email Fender Consumer Relations (the link is under the "Support" tab on the website) and they should be able to tell you the original specs of the guitar. Not sure how far back they can go, but they should be able to go back to at least the mid-to-late 1990s.