Assuming you have a wall with thickness, like a cube you can assign the inside triangles a different material. You can do this in the modeling mode-> attributes menu-> edit materials.
1 add a new material slot
2 assign materials to the different slots so you can see the results
3 select the triangles you want to assign a new material to
4 select which material you want to assign
5 click on assign active material
6 click accept
