"I don't have the right color" every pastel painter has said this, but when you're newer to the medium and still building your collection, you probably really don't have enough colors and need additional pastels to create a more well rounded palette. Deciding which pastel sets to buy can seem like a daunting task, but the good news is- the answer is right at your fingertips- the pastels you already have will tell you which pastels you need.
First things first, I'm going to talk a lot about color value and temperature, if you're unfamiliar with those terms, read these posts before continuing: What is Color Value, and What is Color Temperature. If you're clear on those terms read on.
I've organized the information into categories based on the number of average-sized sets that makeup the pastel collection, but these categories should be used as a loose guide. If you have just one set of pastels, but it happens to be the manufacture's entire collection containing hundreds of sticks, your palette may be more complete than someone who has four smaller sets of pastels. So I suggest reading each paragraph, even if you have more sets than it describes, because you may find that the advice is still applicable. If nothing else- the information can be used as a benchmark, to be sure your palette isn't missing any large segments.
If you have one to three pastels sets: You're probably missing, neutrals, light-values, and dark-values. That's because most beginner sets focus on getting you one or two colors in each color family, and the sticks in those sets also tend to be bright mid-value colors. Get your neutrals first because they really are the backbone of most paintings, then your lights and darks. When I say neutrals, I don't just mean grays and browns, I mean colorful grays and browns too. You need neutral versions of every color (e.g. gray-blues, gray-greens, brownish-reds).
If you have four to five pastel sets: Evaluate your pastels with a color wheel to determine if all the color families are represented, and if there's a full value range.
Check to see if you have reds that range from light, almost-white pinks, to dark, almost-black reds? Do the same for purples, blues, and greens, etc. Go all the way around the color wheel until you've done this for each color family. Don't forget neutrals! This process is easier if you store all your pastels all together in one box or tray, sorted by color/value.
If you keep your pastels together in their original boxes as sets, evaluating them is a little harder. In this case I recommend making your own pastel color wheel so that you can see all the colors together by value. To make a color wheel get a piece of pastel paper and go through your pastels one color at a time. Make a mark with each yellow pastel you have going from lightest to darkest, once you've done this with all the color families you can see if there's anything missing in the value range for each color family.
You have a larger pastel collection (6+ sets): If you have enough pastels that you have the entire color wheel from light to dark represented as described above, it's now time to consider color temperature. Go through each color family as described above but this time check to see if you have warm, cool, and neutral versions for each value range in every color family. For this one you really do need to be storing your pastels together in one tray or box, not in separate sets. (It's also easier to paint if everything is stored together and sorted by color and value so break up those sets!)
More Tips:
Deciding what to buy first: If you know you need more than one value or color group but can't buy them all right away, giving some thought to what you're likely to paint can help you decide what to buy. Here's an example: After evaluating your pastels you find that you're missing lights and dark values, you could also use more blues and greens. You can buy one set right now, and another in two months. You'd like to paint snow scenes, you've also signed up for a sky workshop in three months time. Having light pastels is essential for snow scenes so buy them right away. Then get the blues so you have more options for the sky workshop (the light values you already bought will also be useful for clouds).
When you "don't have the right color" write it down: I keep a list near my easel. If I'm struggling to find a certain type of color (e.g. a dark-green that's the right color for pine trees) I write it down. When I'm ordering supplies I take a look at the list and see if I can find some single pastels that fit the description of what I'm missing.
It's nice to have a variety of textures: Soft pastels range in texture from hard to soft (which is often described as "buttery"). It's nice to have a mix of textures at your disposal, because they allow you to make different types of marks and get different effects. So when you're considering new sets, give some thought to buying a different brands so you'll have a variety of textures.
Final Thoughts: I'm often asked by my students what pastels they should buy next. As much as I wish there was one easy answer I could give, there isn't. It really does come down to what you have already, and what you want to paint. If you're not sure about subjects, then build your palette according to the color wheel as described above, starting with neutrals and light-values. But if you're taken with a particular subject and want to concentrate on painting that, then make sure you have what you need for that subject first. If you want to paint nocturnes, having full range dark-values will be more important than having light-values. In this case, let your artistic desires be the guide!