The problem with Coriander

About a month ago I bought a little coriander plant. It was happy enough, small and green and quite cute. However, pretty soon after it moved into its new home a strange thing happened. The leaves started to change from green to... red. A weird, browny-red colour. It doesn’t look right and is bugging me to no end.

Is it diseased?

Suffering from a nutritional deficiency?
Lack of sunshine?
Rebellious?
Dying?
Really, really cold? I suspect this might be the case.

Unbelievably, The Internet has no information about coriander with funny red leaves. I’ve tried different searches, but only keep getting recipes. I never thought the day would come when The Internet would fail me like this.

I’m hoping the plant will perk up and become green again as we approach spring but it has a long road ahead of it...