Green and peach are colour correctors. Green cancels out redness or ruddiness and peach cancels out purple, like under-eye bags. There are a few more shades used in colour correcting, but MAC doesn't seem to make them anymore (I could swear they used to make violet?).
You mix it into your foundation or concealer just enough just to give a faint tinge of the correcting colour, not to make the concealer lime green or hot peach. It takes a bit of practice to get the mix right, I've had a few green colour correcting disasters back when I first started using them.
I use the peach one everyday to cover up my dark undereye circles. Love, love, love it. I really like the formula of these, because it is thinner than the typical concealor. It doesn't feel heavy or greasy and doesn't settle into those little lines under your eyes.
The green is a great cover-up for zits, too.
I use them directly on the skin before foundation. I haven't tried mixing them with foundations.