Carl T Armenoff wrote a paper on creating oxide films on inconel. There is a table on page 3, the sixth page of the PDF, which gives temperature and colors. On page 9, the 12th page of the PDF, he shows how thick a layer is by color.
This paper inspired me to find a source of inconel; as it turns out, it takes color nicely.
I wanted to contact him to chat, but it turns out he died in 1997.