Color has three components: hue (is it blue? red? green?), value (is it light or dark?), and saturation (is it bright or muted?). To help students understand saturation or intensity we made a 12"X18" scale, with shapes drawn on top. Students chose a set of complimentary colors and put the brightest of each on either end. You can't make a bright color any brighter but you can dull it or make it less intense by adding a little of its opposite. So, for example, a little red would be added to the green, and a little green to the red for the stripes next to the outer, bright stripes. and then a little more of each opposite would be added until a murky brown is in the middle. The positive shapes had the same scale going the opposite direction, so if the background went from green on the left to red on the right, then the foreground would go from red on the left to green on the right.
It is harder to mix colors in gradual, even steps, then one might think. But this sort of sensitivity to variations in intensity can only be learned through practice.