All of my jalapenos have turned red.
Why, you ask?
Jalapenos will start to turn red the longer they are on the plant, and the later it is in the season. Also, the red jalapeno peppers tend to be sweeter and not quite as hot. Mine started to turn a few weeks ago, and I noticed a grower at the farmers market is only bringing red ones as well.
The reds are great to pickle, and even stuff and roast. Yum!