Looking at purchasing my first AR15.  Found a postban colt that I like except it has an A2 upper as opposed to the A3 (which is what I am looking for).  Can I buy an assembled RRA A3 upper (or any other manufacturer for that matter), put the colt barrel on it, and mate it to the colt lower?  In the end, are all of the various manufactures of ar15 parts (RRA, bushy, oly, dpms) compatible with the colt?  I'm new to all of this and trying to learn as much as possible so thanks in advance for all of the help.  

Yes you can get a m-4 colt upper if you want to match your lower. Can't gurantee if it will match up. You always take the chance when you try to match uppers and lowers of having to get a gunsmith to fit them for you. Just make sure that you get quality parts if not you will be disapointed. This comes from personal experiance.