Martin,
Layering colors is quite different from tricolor or 4-color bromoil and gives a totally different effect. The book which PE refers to is "Making Color Prints" by Leadley and Stegmeyer - Little Technical Library, Ziff-Davis Publishing Co. - 1941, and long out of print. I got my copy on ebay. Producing 3 or 4 color bromoil prints is accomplished by the bromoil transfer process using either three or four separate matrices in perfect registration and requires an etching press. In the "good ole days" photo film was used for the matrices and since there was no shrink-swell factor, registration using inked film was a snap! This method, as described by Leadley and Stegmeyer, is no longer possible as film can no longer be used for inked matrices. It won't work! The alternates are to use fiber base "bromoil-friendly" papers and cross your fingers a bit re the registration due to shrink-swell ... or to use RC papers, which is tough, at best. My opinion: it's a quest that "ain't worth it!"
Should you continue on the road to 3 or 4 color bromoil prints, good luck!!!
Cheers,
Gene