It seems like you have a big mess in your head. When people talk about diffuse surfaces in CG they usually mean ideal diffuse surfaces. That is, the brdfBRDF is simply: $f(\omega_o,x,\omega_i) = const$.We're We're talking about Lambertian reflectance. The colour you perceive is due to both the initial light colour and the albedo of the material. If you shine white light, then you'll really perceive what's left of it after absorption. So when you set $albedo = (r,g,b)$ then your surface reflects $r \%$ of the red light, $g\%$ of the green light, and $b\%$ of the blue light. Note that the tricolor formulation is not how it actually works in the real world, to be precise you should really be dealing with wavelengths, however $RGB$ is a good approximation perceptually due to how the human visual system works.
The cosine term is not actually part of the diffuse BRDF either, it comes from Lambert's law (the cosine term in the rendering equation).
Lastly you should realizerealise that even Cook-Torrance is an approximation to a class of semi-plausible BRDFs.
