Vivid COLORS and Dull COLORS
Saturation varies horizontally

Going back to Yellow, 
how do you compare the Yellows 
of a lemon and a pear?

You might say the Yellow 
of  the lemon is brighter, 
but more to the point in this case, 
it is vivid, while the Yellow of 
the pear is dull.

This is an other big difference, but this time one of color Saturation or vividness. 
This attribute is completely separate from those of both Hue and Lightness.

 
SATURATION or
CHROMA
varies horizontally