UPDATE:

The other day I was surfing the internet and I found this specialized painting colour wheel, it shows how real paint colours relate to each other. 

Outside: the purest/brightest colours from the tube.

Inside: naturally muted or earthy colors, like browns and ochres.

The Center: dark neutral tones used for mixing shadows.

The Lines: two equilateral triangles. One shows the triad of primary colors, and the other shows the triad of secondary colors. Triads are traditionally used to create vibrant yet perfectly balanced compositions.

Complementary colors: are those located directly opposite each other on the wheel.

Shadows I:

If you want to achieve rich, dark neutrals, the standard rule is to mix a color with its direct complementary (opposite) tone. However, doing this can often make your shadows look flat or muddy.

Instead of using the exact complementary color, the real trick is to use one of the colors right next to it (either to the left or the right). By doing this, you avoid a dull mix and introduce a beautiful, subtle color bias into your shadow, keeping it clean and full of life. You can see an example of this in the second image.

I want to share this with you because I think it is really illustrative!

the color signatures of various elements when ignited