“Obtains” in its old-fashioned sense means “to prevail, or to gather in strength.” Another way to put it is: “Color increases in saturation in the light relative to the shadow.”
In the Vermeer painting of the milkmaid, the colors of the yellow bodice, blue apron, and orange pitcher are all relatively more saturated in the light than in the shadow. This happens because the color receptors of our eyes respond better in relatively higher light levels.
I was chanting the mantra “COLOR OBTAINS IN THE LIGHT” (much to the befuddlement of passersby) while I was working on this plein air sketch of a sign. Where the light struck the top, I observed that the red was truly much more vibrant.
Like all hard-and-fast rules, this one has a couple of exceptions. When a form of a given color is in shadow, sometimes the shadow is filled with reflected light of a similar hue.
For example, the shadow side of this ochre-colored building in Toledo, Spain is drenched in light reflected from an orange-colored building across the street, making it more saturated than the same color in the full sunlight.
Another exception is that the glare of the sunlight can be so bright that it actually overloads the color receptors of the eyes, draining the chroma from the lights, at least temporarily until the eyes adjust. This often happens in contre-jour light situations.
Addendum: April 7
I contacted Dr. Jack Werner of the Werner Lab of Vision Science, Department of Ophthamology & Vision Science, University of California, Davis, and asked him to clarify these points from a modern scientific perspective. He kindly responded:
In other words, if I dare paraphrase into my own words, we can't evaluate the colors in isolation if they're appearing in a complex scene, because we perceive colors in relation to each other, and the perception of adjacent colors in the light and shade have complex series of effects on each other.
The change in response of the photoreceptors increases with intensity and then simply levels off. I think the saturation changes that you observe in illuminated spots vs. in shadow are rather complex. It is possible that the lightness in one patch induces darkness in the other patch and vice versa. As a result, your observation depends upon both the direct effects of light and the indirect effects due to lateral interactions in the visual pathways.
Thank you, Dr. Werner.