My understanding is that there is no broad standard for line colors. That said, at the club where I sail, here are the colors that are used slightly more often than not:
Main halyard - white with blue/red flecks
Jib halyard - blue with white/red flecks
Spin halyard - white with red flecks
Main sheet - red, orange or white
Jib sheets - usually blue
Spin sheets - red and green to correspond to port/starboard. These colors are reversed for the one boat with separate sheets and guys.
Twings - white with red flecks
Topping lift - purple
Foreguy - yellow (these fade quickly and tend to look white, though)
I think the main value, though, is to have the colors different so that when you bring a non-sailor aboard you can identify a rope by color instead of function (eg, "purple" instead of "topping lift").