I have bought a fairly decent '73 CB750 which is now in two shades of red!!!
Is there any way to find out the factory colours from the frame serial number? 
There's no paint code in the VIN, it's just a straight up serial number. Is there any stock paint showing through on the under side of the tank?
If you're just aiming for authenticity of paint color, any of the available schemes from that model year will be fine. Nobody will ever be able to tell if it's not the same color that it was from the factory, but they will know it's not the original paint no matter what you choose.