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. It is excerpted from a longer and more detailed post "Building Art Beyond the Image"
http://forum.portraitartist.com/show...s=&threadid=58
As a highlight transitions into a deep shadow, warm and cool tones begin to alternate. This creates each layer that defines form. The overlapping of warm and cool color is essential in building realistic form. (The terms "warm" and "cool" color are relative to the specific color used...i.e., warm and cool skin tones)
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. The reflected light should match the value of the shadow and it can be either warm or cool in color.
I like to make reflected light by mixing two color opposites (i.e., red/green, purple/yellow) to neutralize each (can look like mud). Add enough white to this mixture to match the value of the shadow (and sometimes a touch of blue in addition).
Below is a detail of a face by Rubens. With specific reference to the numbers above, I hope you can see the layering technique I have described. If you can begin to see this, you can begin to paint it.