Off the Cuff: Why are my leaves coming in red?

So basically: the reddish tones are anthocyanins, which are the reason a lot of plants in full sun come in pink, red, or purple (or -toned, at least)!

They're produced over chlorophyll for baby leaves, because until they harden off/mature, they don't need to produce energy really — the old leaves are doing that — but the old leaves are also sensing how much light they're receiving (which if it's sunlight, means how much UV radiation they're also receiving).

UV radiation messes with everything, and chlorophyll doesn't protect against it at all — but anthocyanins do!

So while it's hardening off, it's red with protective anthocyanins, and then once it's sufficiently mature, the plant automatically recycles the nutrients from the anthocyanins and produces its normal chlorophyll, which turns it green.