In my article, electrical wire sizes, I break down the common wire sizes used for everything from feeder wires, to branch circuitry, to low voltage uses. The numbering system of wire can be quite deceiving, unless you know what the numbers mean. A smaller number gauge wire has a larger ampacity, which is probably reverse thinking for most.
Along with wire size, you'll need to consider the wire cable length. In long runs of wire, the resistance of the wire acts like a load, using some of the capacity of the wire load. Thus, a wire capable of carrying 200 amps, may only be good for 180 now. In this case, the next larger wire should be used to allow safe distribution of the load.
To some, wire color means nothing. As long as it's the right sized wire, what's the difference? Well, as an electrician, I'll tell you that I don't like finding green wires connected to circuit breakers, and I have in an old school I worked in. You see, green is for ground wires, at least usually. Knowing which wires are used for what makes for a less complicated installation. In fact, here's a video to show you what the different colors mean. Play Video.