It is OK to paint what you see, but it is much more important to paint what you know. The human eye is not always "sophisticated" or "trained" enough to observe reality and the Old Masters certainly knew this.
Here is a quickie visual lesson in layering warm and cool paint to define realistic looking form. The example with numbers superimposed below is a head painted by Rubens. (Wisecracks about "painting by the numbers" won't be appreciated).
As light strikes a form most intensely at the highlight, it will transition into a deep shadow. As the light flows around the form, warm and cool tones begin to alternate.
1. Highlight is cool. The lightest value, cool color paint on an object.
2. Light is warm. The next lightest value, warm color paint - and it continues to get lighter still as it approaches the area of highlight.
3. Halftone (where light and shadow meet) is cool. A mid-value, cooler color paint where light begins to turn into shadow - but can't be defined as either light or shadow.
4. Shadow is warm. A dark value, warm color paint.
5. Deep Shadow (cast shadow at the origin) is hot. Darkest value, hottest color paint.
6. Reflected light within a shadow is as close to pure color as you can make it (see the post just before this one for more detail). The reflected light should match the value of the shadow and it can be either warm or cool in color.
The above illustration is merely meant to help the beginner "see" and is not meant to be a set of rigid rules.
So many begining painters "crash and burn"...sadly, left alone to suffer from an untrained eye. Unless they are taught to "see" they will never learn to paint well.
I feel that any help a beginner can get to produce a good result is valid. With a few successful paintings under a belt, so to speak, a beginner can gain the knowledge and experience that works for them.