I've got one in my front yard that is doing this, while the one in the back yard isn't. I'm fairly certain its just a stress reaction though, as we had a cold snap a few days after I planted the one up front.

Since you're a zone above me, I'd wager you've had some cold spells as well. The plant is probably just a bit confused, thinking its fall and is changing colors. Soon as it warms back up and stays that way it should go back to being green again.

I've got a number of herbs, catnip included, that turn purple when it gets too cold for them.

FataMorgana