Chuck is spot on here. The primary purpose of the Cyanide treatment was for that "Hard" skin of high carbon which had been cooked into a piece of low carbon steel via the molten Cyanide bath. The colors were secondary. This was a quicker & less expensive method than Pack Carburizing & was useful for many purposes where only a relatively thin skin was needed. By using longer times in the furnace a deeper skin could be attained by the pack carburizing process than by Cyaniding.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra