You can hatch colours together by making lots of marks with the end of the pastel. As you lay marks of different colours over each other, they blend together, but you can still see all of the colours showing through in places.
You don’t have to make the marks go in one direction. This is a useful and versatile technique for animal work, portraits, landscapes; in fact, all sorts of things.
Pastel grip for hatching, to make marks with the end of the pastel…
The marks don’t have to be straight; try making lots of little marks going in the same direction, or in lots of different directions, or following the contours of the object you are drawing. Use lots of colours and overlap the marks, seeing how they blend without being smudged.