There's an article explaining what a 'diffuse surface' (or material) means: https://en.wikipedia.org/wiki/Diffuse_reflection And the classic: http://www.huevaluechroma.com/021.php
Hmm, man i'd say that he needs to learn how to render diffuse colour simple materials before he even begins to think about things like fresnel, metallic specular and interference. You start with the simple case, nail it, then move on.
You can say you disagree with me, but it's just not how the physics works out. Full disclaimer, i program a tool that pretty much does nothing but make coloured spheres for generating light based colour palletes called colour constructor, so we can get into the maths if you want. The saturation drop you are seeing is not…