Working with a limited palette is great way to understand color theory as well as to keep your paintings unified and cohesive. In pastel, this is a little more difficult to do than in paint because we optically mix the colors on the paper rather than physically mixing them on the palette. A good exercise is to paint a predominantly green landscape using only blue and yellow pastel sticks to make the greens.
In the plein air study above, I used a variety of blues and yellows, plus a stick of burnt umber with a reddish cast. I started by doing an underpainting in the reddish burnt umber to work out the values. On top of this I went in with a Sennelier dark brown-black soft pastel to establish the darks. Next, I went to a Prussian blue hard pastel to begin working through the middle values. From there I worked through the middle values using an aqua blue and a violet blue for variety, plus a pale blue on the brightest areas. On top of all the blues, I added in various shades of yellow to create a variety of green hues in different values. The fact that you are looking through one color onto another to create a third color makes the end result more atmospheric than if straight greens were used.
Working this way has several advantages: less pastels to buy, less to carry and a cohesive look to the painting. You’ll find you can get quite a range of greens by working in this manner. You can also use reds to tone down any areas that need to be more neutral.