These are color spheres, and on the right are spheres which show the 12 hues of the subtractive color wheel progressing from light to dark. On the left is the inside of the color sphere, which shows contrast in saturation achieved by mixing complimentary colors together. Bottom left is the inside of the color sphere cut vertically, which shows contrast in saturation, light dark contrast, and complimentary contrast. Now, these aren't primary colors, these are subtractive colors.Subtractive colors are mixed together to get primary colors. If you want to mix colors digitally, then you will get the most accurate results by using subtractive colors. This can be done by layering opaque colors and sampling them with the eyedropper as I've done here.
"Generally speaking, sensitivity to color is biased in the same way as subjective taste. Persons of a blue subjective timbre will perceive numerous variations of blue, but possibly very few of red. It is therefore worthwhile to carry out the [color] exercises through the whole domain of colors. In this way we become more just in our evaluation of color." Johannes Itten