I have two decorative kale plants that are turning blue. They are in their original pots and didn't turn blue until a couple of weeks ago. They've been outside all winter, and I've had them since October. Does anyone know what is causing the blue coloring? I've tried Googling it to no avail.
Kale turning blue
No, I do not know why they should start turning blue.
There are varieties with blue leaves, but I think in your case it has to do with temperature... and a higher level of Anthocyanin, the chemical that gives it color.
"Color Reversion. Color development of ornamental cabbage and kale is due to the accumulation of anthocyanin in the foliage. Anthocyanin, a pigmented flavanoid, is responsible for most of the red, pink, purple and blue colors observed in plants. The synthesis of anthocyanin in ornamental cabbage tissue is due to the cessation of chlorophyll production when temperatures drop below 55º F. Once temperatures become cool enough, the upper foliage will develop brilliant colored centers. Sometimes, the upper central leaves can undergo a color reversion if warmer temperatures return. Because temperature control is impractical with ornamental cabbage and kale production, this problem is unavoidable."