Why does the artist want Griet to look at the cloud?

In this section, Vermeer is attempting to explain color. He asks Griet to look at the clouds and tell him what color she sees..... Griet, of course, says white. Vermeer then shows Griet the reality of color...... the blues, yellows, grays, and greens.... the way they all work together to give the illusion of being white. He then explains that this is the model he emulates in his art..... the reason he doesn't need blue paint to create a blue skirt.