Certified GKH Gardening Expert
This is because the plant cells in the leaves become punctured by ice crystals and leak out water. Think of it like a piece of cloth, when it gets wet, the color of the cloth appears dark. This is exactly the same with the leaves.
After the plant cell walls are punctured, they cannot support themselves and start to collapse and die, which is the wilting and turning brown.
Plants that are able to survive outside in cold weather produce an anti-freeze in their cells that prevents the formation of ice crystals and therefore prevents their cell walls from being punctured.