Try to mix Cobalt blue and cadmium red to a purple color, it is not possible, although blue and red should become purple in theory. Then try French ultramarine and Alizarin, the mixture is great. Why is this? Here is an explanation.
In this example, I have mixed orange, but it works with all color mixtures. It does not matter what colors I have used, use the ones you have. All lemon yellow works, it must not be “Lemon Yellow”. Same thing with the other colors, all warm yellow, cold red and warm red work well.
Lemon Yellow from Daniel Smith (PY175)
Quinacridone rose (PV19)
Result: a rather brownish mixture.
Nickel Azo Yellow from Daniel Smith (PY150)
Quinacridone Coral (PR209)
Results: Significantly brighter orange than the above.
The result of the above mixtures is that cold yellow + cold red (PY175 + PV19) gives a less pure mixture, than the one between warm yellow and warm red (PY150 + PR209), why is this?
This illustration with the color wheel explains why this is the case. The warm yellow and the warm red are right next to each other in the color circle. While the cold yellow and the cold red are significantly further apart,
With two pigments that are close to each other, a mixture is obtained that is closer to the periphery of the circle and therefore a clearer mixture is achieved than with two pigments that are further apart.
The closer you get to the center of the circle, the more blackish the mixture becomes (in the center is black), the less clear the mixture becomes.
In other words, if you are trying to achieve a pure mixture, use two colors that are close together in the color circle. However, if you want to achieve an unclear result, choose two colors that are far apart.