Paints normally contain a vehicle- a liquid that holds the pigments, and hardens after painting, and pigments, that are the actual color-producing part. Pigments are a very general term, and can refer merely to those materials folks can buy to make their own paint, or to add some color to some other substance like plaster, to give it color.
Folks have found that one can apply pigments directly to a model to give the same effect as drybrushing paint.
I have no idea what the definition of pastels are as a substance used in model building. I think of pastels as colors of very low saturation. Colored chalks are an example.