I'm trying to follow a LightWave tutorial with C4D, and am having trouble figuring one step out. They use a vertex map to control the specularity of a certain area: a character's lips. I.e., the material is the same skin color but in the area of the vertex map it should be shiny. Fairly matte otherwise. They're using the vertex map to affect LW's Specularity channel, but C4D doesn't have a place to put a map/shader in the Specularity tab.
The Diffusion channel with "affect specularity" checked seems to partially work, but when I tried to put a black-and-white checkerboard in C4D's Diffusion channel as a test, in addition to affecting its specularity it also darkens the black areas. Not what I want - the test plane here should be fully red, but with only the specularity showing a checkerboard pattern.
I'm obviously missing something, but what?