Its been happening to me too, I'm closer to St Louis, I assume like budbro, that you are outside. Its actually a form of stress that causes the purple tones, because it is too cold. Lots of plants do it, my morning glories and trumpet vines have purple petioles and purple in their leaves too. I can only imagine how cold it is around chitown, I went there one year in march/april, and had my heavy winter coat. I was still cold lol