Adding very dark colors

[xigoi]

The palette for each hue includes light colors that go almost all the way to white. However, the darkest colors are still quite bright. How would one go about extending the palette to the right to add darker versions of the colors?

[fchristant]

Have a look at chapter V of the original article, it explains how to add your own hues in a consistent way:
https://ferdychristant.com/color-for-the-color-challenged-884c7aa04a56

However, as you look add extra tones to the existing hues, it's probably best to use the Photoshop picker. Dial in the values of the current darkest color of the hue, then change the "B" in the HSB color system. You can also play around with "S" but for very dark colors it will not do much.

Hopefully this helps?