I want to talk here about what I see as soft pastels' biggest advantage over other mediums:  Depth of colour.

Consider how the human eye sees colour:  When looking at,  say, a leaf, rarely does the eye see only "green",  but different shades of greens.  Some areas of the leaf may appear to be a perfect balance of green, others a bit more yellow,  some darker, bluer, cooler, warmer, more olive-  how does an artist capture this amazingly rich and varied depth of colour?  To me,  the answer lies in the layering capabilities of soft pastel.

Pastels are an opaque medium.  This is important to keep in mind because it is the basis of rich colour technique.  Consider:  In liquid mediums,  colours and values are mixed on a palette- a pile of red, a pile of blue, a pile of yellow, a nice-sized dollop of white, a smear of black- and in-between daubs of mixtures making up the rest of the colourwheel.  These little daubs are now transferred to the ground where they are generally blended some more so that one hue melts into another either by physical blending with the brush, or the chemical properties of the medium.  With pastels,  while the artist can certainly blend (using fingers or some other tool)  the "more natural" appearing work is to simply layer the disparate colours over one and other, allowing them to "blend themselves" optically rather than chemically or mechanically.    Let's look at some examples.

This piece is a quick study of various copper items I have sitting around.  In grayscale, you can see the values are correct and that everything "looks metallic":

Had I used only various brown and oranges, though, while the values still would've been correct, the "metallic look" would be missing,  and the pieces would probably not look as much like copper.  This is because copper has a tendency to "shade" into purple.  Well,  orange and purple are nearly complements;  in liquid mediums, mixing one into the other is a sure-fire recipe for either graying the colour (lowering chroma), or making mud-  and copper suffers from neither effect when seen in reality.  With pastel, though,  I can layer purple over orange, skim another layer of orange on top, and achieve a realistic appearance of shadowed copper.

Note the lavender on the back pot's spout,  and the darker purple shadowing the teakettle's front-  it even shades to a green around the edges,  and the shadow reflects both the warm peach glow of the metal, and the lavender and gray which say "shadow" to our eyes.   It is layering- without finger or mechanical blending-  which allows this techniique to work.

Next I want to touch on differing colour wheels, and the way I think about colour.

Demo:  Fruit and Cyclamen Still Life

Colour Wheels and How They Spin