I started growing my cherry tomatoes in July, but used a container rather then planting them in the ground. Most of the tomatoes have ripened. But I have a few that are not turning red, but staying purple. A few questions:
It sounds as if you've planted a purple variety that turns red when it ripens (I plant a non-cherry variety called Blue Beauty [at least I think that's its name], which does exactly that). If so, these may not turn red due to reduced light levels (and perhaps cooler temperatures), but they'll ripen anyway. I'd either pick one now and try it and/or pick all of them and put them in the house to see if they'll "redden up" over the next few days.