I didn’t mean to overemphasize color, though. Although color contrast is important, the bulk of the design principles in the book I referenced address form, structure, size, etc. I only made the extensive reference to dress and color because it’s one medium where we all automatically get it. No one puts on army green pants with a pea-green top. We naturally choose contrast. In our document design, we should employ similar principles of contrast. For example, a larger title in a completely different font, and so on. We have to exaggerate the contrast even more than we’re accustomed to.

Color contrast is a complex and very powerful tool. You did a good job of briefly explaining the most salient points of that tool.

As you and the author say, keen awareness of and continuous use of color contrast in photos and fonts is critical to helping readers better understand what you are trying to convey.
And…yes…color contrast, despite the quality of the content, is often the difference between a piece that succeeds or fails, .
Look at the contrasting colors of birds in your own backyard for proof of the importance of color contrast in nature.

One minor observation: I have difficulty with the illustrations of the guy running on the treadmill.
In both examples, the large red and black oriental rug in the background is too busy and loud. It draws your attention away from the guy’s contrasting clothes colors. It also overpowers and bleeds into the orange, black and red colors the guy is wearing. This makes it difficult to clearly see the conflict and contrast.
If the wall in the background was blank it would be more neutral and allow the contrast to be clearer.

In contrast, the photo of the three young girls makes the point much better because the background is more neutral.