The trick with mixing colours is to to do a small tester first as when you do your rail laps the chances are the colours will have completely mixed together and the lap will be a solid mixed up colour. For me once red, orange and yellow almost became brown and i'm not too sure what blue and green will make.
Then i did blue and yellow and surprise surprise that made green, see photos.
It wasn't my best effort but i did learn alot about the unpredictability of resin swirls.
I'm a big fan now of only mixing different shades of the same colour. Have you been to austin surfboards website? They are the absolute masters of resin swirls and tints and will usually answer any emails you send them re glassing they're really helpful to home builders .
http://www.austinsurfboards.com/
Hope this helps.
KS