To find a true intense color, look to the outside of the color wheel. All the colors on that outside ring are true color. As a complement or a split complement is added to the color it will begin to gray. Earth tone colors are variations of this phenomena. For example, ochre is a grayed yellow made by adding purple, blue purple, or red purple to the yellow. Burnt sienna is made by graying a red using its complement of green. Orange is grayed by blue - a combination that creates a really wonderful brown.
The degree of neutral is determined by how much of the complement is added. As the complements near equal amounts in the mixture, the color becomes gray. Adding more of either side creates more of a brownish hue.
I'm also including this week two picture I painted of the rocks in Sedona. They're done in watercolor on foamcore so they are very loose.