Vang,
Zinc Cromate was an anti-corrosive protective coating used on many parts of US aircraft interior structures during WWII. The color varied depending on which parts of the airframe were being coated. Some structures were a bright green (like wheel wells), some a more bronze green (like some cockpits) I don't know enough about the systems used to give a definitive answer about what colors were used on every type of aircraft - try asking a propellor-head in the Aircraft forum - they'll know there.
As far as painting goes, I try to paint as little as possible - but let me clarify.
On cars, for example, there may be several components on the engine or drivetrain that must be glued together, but are to be painted the same color. I usually assemble what can be assembed, then paint it all in one hit - instead of painting every single bit separately (lots of work) and THEN assembling them - get it? Doing it this way, you'll reduce the chance of messing up a nicely painted part with glue.
Try to make sure the all the bits are painted before final assembly - you might find it hard to paint something if 27 other things are already glued to it.
Another point - try to use as much patience as you can - doing things too fast (like not waiting for paint or glue to dry properly) only results in a bad finish.
Sometimes you'll have to paint something before you attach other things. Remember, patience and a methodical approach are your two most important tools on the workbench.
If you have any other questions, just ask. You can email me direct, but if you post your question here, you'll stand a better chance of getting lots more answers.
Cheers,
LeeTree