What is the big deal about pure MX dye you may ask? There are a number of reasons I like using them:
1. It's more economical. Premixed dyes also have fillers.
2. It allows you to control the color. Premixed colors may be discontinued. If you know how to make a particular color, you will always be able to make it from the pure colors.
3. Working with the pure colors helps you understand how the Procion MX dyes interact with one another. Because they are transparent, they are affected by any underlay, as well as anything they are mixed with. It is not the same as working with paint, particularly if you use low water immersion techniques.
4. Some colors have particular requirements. Turquoise, for example, is a picky little molecule. The reds strike quickly. If I have mixed the color myself I know which pure colors went into the mix and what their particular requirements are for batching etc.
The MX names give you clues about the color and strength of each color, but I am too lazy to learn them, they just don't stick in my head. Paula Burch, ever a font of MX information, has a detailed explanation here. She also has a great chart showing the different names for each of the pure dyes by supplier. Seems like turquoise is the only one they can agree on! When someone mentions a particular color, ask who the supplier was or you may end up with the wrong color.
Here is the difference between pure and mixed colors. I was using up old dyes, so I dropped some powder in the sink and ran water over it.
Here is a Dharma's navy. It looks like just a very dark blue in powder form, but when in liquid it separates and there is bright pink too!
Contrast that with the pure dye, grape, where the same color exists in every dye molecule, no splitting into other color components.