Here is a color chart link....when making a chisel I would run the colors until the cutting edge was purple..if tempering a lathe bit, I would run the colors until it was straw colored. Everything will be tempered to a different color, just depends on the hardness desired. The really hard things like a lathe bit will shatter if it is smashed and of course a chisel you don't want to shatter as you are using it. The brittleness goes away with the higher temperatures you temper it at...and as you look at the chart..the further you go past the yellow and towards the blue. One thing I noticed on the last link I posted is that they were not quenching after running the colors..I always quench at that step to stop the running colors. When you run the colors, use minimal heat, as it is easy to go past the color you want!