[paul.fr]

Why does steel change colour as it's heated, and what happens to it's structure?

[lightarrow]

Why does steel change colour as it's heated, and what happens to it's structure?

The colour is due to the formation of thin layers of oxide; the exact colour depends on the exact film's tickness: see interference by thin layer or similar concept. There is something here:
http://en.wikipedia.org/wiki/Anti-reflective_coating