Okay, stupid question. How easy can a dog catch a cold in snow?

I have a 4 mo. old ( 7 lb.) Jack/Chi mix. I'm not sure how long it's appropriate to let him play in the snow because I don't want him to get sick. I am using dog booties, and I keep him warm in a snow coat. He is an indoor dog but I take him for walks and to our local park. We're usually gone for roughly 2 hours. He is never unsupervised. How long can he be outside before he's in danger from the elements?