When working with pastels, you don't mix shades ahead of time on a palette. To create green from yellow and blue, for example, you will need to blend the two primary colors together directly on your work.
What you need to know
Blending colors is usually done with your finger. Stumps are reserved for small areas.
Similarly, by varying how much of each color you use, you can create subtle shades.
Spread the first color as a solid area of color or as widely-spaced, not overly dark hatching.
Then overlay the second color the same way.
Work together with your finger, in a circular motion: the pigments will mix to create an intermediate shade.
To optimize your chances of obtaining the shades you need, practice on an extra piece of paper with the same color and grain.
Apply the first layer of color, drawing hatching from left to right. Gradually increase the density of the pastel, by, for example, drawing the hatching closer together.
Spread the second color in the opposite direction: apply the least pigment where the first color is most intense.
Blend the two colors using the tips of your fingers, always working in the same direction: never go backwards! The result will depend on the direction in which you blend the colors. From light to dark: this will give you a more luminous gradation; from dark to light: the overall tone will darken.
Of course, a single color can also be gradated. All you have to do is increase the pressure on the stick as you work your way across the sheet of paper, from left to right. You can also do it the opposite way: start by pressing down hard on the stick and gradually lighten the pressure.
