It sure is acting like my Chocolate Mint does. Mine changes colors in winter, that is only the parts of the plants that are not in a protected are. It has the same runners that mine have. I have have my plant for ten years. It of course has grow through the planning bed where I have let it. My orange mint did all the same things except die off for winter. Both my mints have different shaped stems depending on which part of the plant you feel. I don't really know what it is but the pictures in the summer should definitely give you a solid answer!
