Not lettuce, appears to be a brassica. Some Bistro mixes contain Asian greens.

You pic is not very clear but it kinda looks like kale,perhaps red russian.

I agree with Itzybitzy. Looks very much like my Red Russian/Ragged Jack kale. I've had it in a mesclun mix from Baker Creek and also grown it alone. I prefer it cooked when it gets larger.

Definitely red russian kale. Stir fry it with a bit of garlic, salt, olive oil and hot peppers. Delicious. Very nutritious green, head and shoulders above lettuce. Very young leaves can be used in salads.
I'm not sure what elevation you are in Colorado but in my zone 2b, kale continues growing until Halloween which is 6 weeks after first frost.