This might come in handy.

[IanBryant]

I came across this at work and took a picture to reference later and thought it may come in handy for some forum members too so I'm sharing.
It's a chart of different colors of carbon steel at different temperatures. I'msure it would serve as a good baseline but there are bound to be variations, especially with tool steels.

Sorry about the quality, the paper had seen better days so I photocopied it...then took a pic..

 

[Doug Lester]

I've seen that in various publications and I personally consider it of questionable value. There are a variety of things that can interfere with the oxide colors that occur at tempering. This is one place where a regulated oven, even a toaster oven, is far superior.

Doug