Here’s the PDF for how CDProjekt is doing their terrain. Might give some guidance or help a light bulb appear above your head . It’s not a tutorial. I’m no pro myself so I’m interested in learning. They’re doing exactly what you want. Specifically detailed in the “Pigment Map” portion of the presentation. It’s certainly interesting and hopefully some experts chime in to help translate CDProjekt’s method into a BP method.
Anyway, the PDF link: http://twvideo01.ubm-us.net/o1/vault/GDC2014/Presentations/Gollent_Marcin_Landscape_Creation_and.pdf