In regards to the limits of primitive numbers and texturing/color, I was thinking to focus the challenge to certain aspects of composing a scene. If you had unlimited primitives, you could just mash a thousand spheres together and create a fluid looking organic surface, or design a highly detailed hard surface with hundreds of cubes and cylinders. That's not the point, though I'm not trying to be a dictator or anything.

As for texturing, I think one material works fine. Use the lighting instead of the textures to define the value of the objects. Value being the tonal range between black and white. If you'd rather choose a different material, that's fine, but keep it as a single greyscale tone for all the objects. (e.g. a dark grey blinn)

Personally, I find that some constraints enhance creativity. This is a challenge after all.