The color you see on the on original parts is not meant as a finish, it's actually a byproduct of the tempering process they used at the factory. The good thing about it is that the color is a great temperature indicator, so if you heat it to the factory color you won't exceed the temperature the factory used to temper it. You can do any part like that without hurting it, and some say you might even prolong the life of parts that have become "work hardened" by years of use. However, if you heat it too much, you have to reharden and temper again.
Annealing is a totally different animal. This is done to soften the steel before you work on it, and you have to heat it to a much higher temperature (glowing hot). The only color you get is steel grey.
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