There are two main types of parkerizing, zinc phosphate and manganese phosphate. GENERALLY, the zinc produces a light to dark grey and the manganese dark grey to black. There is a pre treatment to turn both black, IIRC it's just an acid solution. The zinc has a finer grain to it and is usually preferred if the park will be painted over. The color of both types is affected by the type of steel in the part, heat treatment, and solution temp and age.