Heimo,
WALL_SKINS_BMAT_NAMES works only in 2d, as it was developed for labeling purposes – it is a bug in the manual that it said it works in 3D too. In a future version 3D functionality can be added to this global. Now I corrected the online version of the reference guide to make it clear.
In 3D you can use WALL_MAT_A and WALL_MAT_B for the wall surfaces.
Gergely Fehér
Team Leader, Library Team
GRAPHISOFT SE