Bill Tindall, E.Tn.
I have planed thousands of bdft of steamed walnut and I have not observed a decrease in heartwood color. Several thousand bdft of walnut will go into the steamer. Some color runs out in the condensing steam, but it is an insignificant amount.
In steaming the condensing steam does extract color from the heartwood. This colored water must do some staining of the sapwood but the depth of staining would not penetrate to the center of the lumber.