In order to create solid compositions, and achieve a sense of realism or believability, you have to have a good sense of value relationships first. Take any painting that has stood the test of time and put it into black and white, and you’ll see strong areas of light and dark.
The above image shows colors, sampled from photoshop, all with 100% saturation, each 10 hue steps from the one previous, for a total of 36 colors. Below that is the corresponding black and white value of the color. It’s clear to see how yellow is the lightest color, and a deep blue hue the darkest. If you were to sample the black and white values, the lightest yellow is nearly white hovering near 90%, while the dark blue is close to black, hovering around 10%.
You can, of course, lighten or darken any color by “adding” black or white to it. In photoshop that means pushing the brightness up or down, but by doing so, you also lose some vibrancy.
I wanted to create this scale for myself to get a better sense of what the actual values of each of the colors is, so that, as I paint, I have a better idea of what colors will work in what areas of my painting, while maintaining strong value relationships. It’s not always easy to see the value relationships when you start to add color after all, so knowing the relative values can help tremendously.