[Karin Wells]

Aaaaaargh! This is getting complicated, but on your example of Guido Reni's work (1575-1642) "The Abduction of Helen"....the blue on the neck is part of an old system of color bands that they used in those days. Here is how it works:

When rendering skin tones of the face, only the color of Yellow Ochre appeared above the eyebrows.

From the eyebrows to the chin, only the color Vermillion appeared.

From the chin line on down over the neck and chest, Blue was the color used.

If any of these colors get mixed up, (i.e., Red on the forehead looks "lobsterish", Blue in the face looks like illness, Yellow on the neck looks "cadaverish"....or whatever) the result is a visual disaster.

If you go to a museum, look at the Italian renaissance masters and you can detect the use of these color bands.

Some painters, like Reni, may have pushed the envelope...

As far as primary colors go, my palette consists of a warm and a cool red, a warm and a cool yellow and a warm and a cool blue (glazes only on blues) plus black and white.

I personally think that if any one of the primary colors (Red, Yellow, Blue) plus Black and White are missing from a painting, the eye will "hunger" and the painting will look as if something is missing.

For example, if you look at each and every one of Vermeer's paintings, you will see the following represented:
Red
Yellow
Blue
Black
White

As far as the Mona Lisa goes, I suspect that you are looking at a poor reproduction...when I saw her two years ago, she was like a little jewel. I was surprised at how much better she looked than I expected her to - she wasn't yellow.