When painting in pastels, it's important to use your sticks in a particular order; the harder brands first, and saving the softest, butteriest sticks for highlights. The softer the pastel, the more pigment it leaves on the paper (or other support) and the more the paper 'tooth' gets clogged with color. If you start with your softest pastels, you'd better put the color exactly where you want it and have no intention of mixing on the support or blending a layer either optically or physically, because you won't be able to get the second layer to adhere.

Learning the relative hardness/softness of the various brands of pastels is an empirical study - you learn by doing. The task is complicated by the fact that within the same brand and even color-range, relative hardnesses can vary.

Still, that said, you can make some generalities about the brands of pastels. Here's my take on the brands in my collection:

HARDEST > SOFTEST

Pastel pencils: Although I typically don't use these in a painting, I do have a collection of pastel pencils. These are all Contes and Derwents. The Contes are harder than the Derwents, even slightly gritty - the noise they make on the paper is raspy.

Hard pastels: There are two main brands of hard pastels; NuPastels and Conte crayons. Contes are harder than NuPastels. Originally Conte Crayons only came in earthen tones such as sanguine and sienna, black and white. Now they come in the complete range of colors. NuPastels have always had a complete spectrum.

SOFT PASTELS:

Most of the pastels on the market are so-called 'soft' pastels, yet their degree of softness is open to debate. Some are harder than NuPastels, at least in some color ranges.

Conte Soft Pastels - these are the hardest 'soft' pastels in my collection. They are hard to find in the US, although more widely available in Europe, I believe. They are also often found on eBay.

Rembrandts - next hardest are the Rembrant pastels. Some of these can be quite hard, while others can be almost buttery.

Winsor-Newton & Grumbacher - these brands are fairly comparable in softness.

Daler Rowney - Now we're entering 'buttery' territory; from here on the pastels are smooth, soft and have a high degree of coverage. Use carefully until you're sure you've got the tones you want.

Unison - The Rolls Royce of Pastels. Handrolled and air-dried, these are very expensive, but worth it. A great balance between coverage and layerability. Unfortunately, I have very few of these in my collection.

Sennelier - These are the brand that Degas used. They are exceedingly soft, and quite brittle, an unfortunate combination. You may find yourself laying down a stroke and find the stick crumble in your hand and very expensive colored dust lying at your feet. Too expensive to waste this way.

These 'ratings' are based on my subjective experience. You may find some pastels softer than I have. Play with your 'colors' - try various brands on top of one another and see how much coverage you can get on top of the bottom layer.