It is important to know how to paint what you see and accurately assess hue saturation and value by eye.

It is important to know the different components of light including Ambient /Diffuse, Bounced Light, Ambient Occlusion, Specular Highlights, Secondary Reflections, Subsurface Scattering, Atmosphere and Fog, and Bloom and Flares.

There is both specular and diffuse reflection and refraction.

I promote the idea of thinking in terms of color temperature where hue saturation and value are all interlocked.

Color relativity needs to be understood in order to properly see subtle colors and that grey is often a transition between other hues.