Depends upon the temperature on your porch. Collards, cabbage, mustard greens and turnip greens all are relatively cold hardy and will live through light to moderate frosts. Depending upon wind, exposure and humidity mine do fine to below 20 degrees. Below that they suffer rather intensely. My neighbors here in Tobaccoville, NC (Yep that is the real name) all keep winter greens growing as long as they can.
If the temperature can be kept high enough on your porch you should have no problem growing collards even in Wisconsin. Do you have a way to heat it or does it have a southern exposure with plenty of sunshine and concrete or brick to act as a heat sink? I use a brick foundation for my cold frame with dark painted water filled milk jugs to store heat and that works fine for spinach and lettuce. We are not in the banana belt here near the Blue Ridge. We are on the boundary between zones 6 and 7.

And the cold is good for the collards, it makes them sweeter. I try not to eat mine until they have been "frosted", but in No. Florida it is a bit of a wait, lol.

Okay No. Florida, you should not have a problem with the frost now, lol. Now you can feel our northern pain.

tatsoi would be another great option.

I planted collard greens in my winter garden. I simply picked of the leaves and more collards continue to grow. It's February now. I know that collards like the cold. Should I pull them all up and freeze them now or continue to eat off of the plant?

I would just continue to eat them off the plant...In zone 6 you should still have some time to enjoy collards.

Kale is the cold hardiest of the cole crops. Other cold hardy greens include spinach, miner's lettuce, corn salad, and winter lettuce cultivars like Winter Density and Rouge d'hiver.