One of the things that draws us to polymer clay is the absolutely infinite color possibilities. In fact, it can be overwhelming to explore something so endless. Some folks love to do that, and others don't enjoy it quite so much. So the clay community has "developed" two basic approaches to the subject. Each approach has its merits and its place in our clay tool box. As of this writing, we have two very different tutorials here on JL, both of them teaching color, and each with a different and highly valuable approach.
One approach is the formula approach. In the formula approach we mix this proportion of one color added to that proportion of another color, and perhaps a specified proportion of a third color. This is the most common approach given in tutorials, by far, because it ensures that you will get a result most similar to the project being taught. It's also valuable to know how many parts of one color to mix with a certain number of parts of another color when you are making your owm color blends, because if you need to make more of that color later on you will be able to replicate it with certainty. Knowing the formula can also save time because you can get exactly the color you want without any trial and error.
The only "problem" with this approach is that if the clays get reformulated (which happens) or if certain colors are dropped from the line (we all know who pulled THAT stunt most recently), you need to go back to the drawing board to determine a new formula for your mixed colors, and you might also need to develop a formula to replace a from-the-package color that was dropped. Or you might even need to switch to another brand of clay in order to get the right combination of base colors for your formula so you can replicate it. This more or less goes with the territory. Figuring out, and sharing, formulas is something the clay community does very well.
The other approach is more "painterly" and more based in traditional color theory. Following this approach forces you to train your color eye to recognize HOW to shift the color in your hand so it will more closely match the color in your mind. Using the color theory approach, you can start to home in on your OWN formulas for shifting the clay from a package color to a custom mix. You still might need to develop an actual formula, so you can precisely replicate a custom color, but understanding the theory will help you zero in on that forumula much more quickly. On the other hand if your clay work is more free and loose, you may not need forumulas so the color theory approach can be very effective and efficient.
The disadvantage of the basic color theory approach is that you can have so much fun shifting colors around, that you never get anything made! The explorer can take over your clay time and at the end, you've got a bunch of little wads of neat colors. And no idea how you made them. So the key to this approach is to infuse it with a bit of discipline and document how you got there. Most people who teach color theory for clayers also make good suggestions on how to structure your exploration just a bit so you'll have a trail of bread crumbs to follow to your favorite mixes.
I think that, over time, most clayers do a little of each. Using formulas that are tried and true by other clayers to help you get started as a beginner is a good way to go. This may be particularly true if you've not had a lot of exposure to color theory. Eventually, though, I think that anyone who clays quite a bit gradually picks up some color theory by accident if not intentionally, and this throws the door open to work with confidence. Confidence, in my opinion, is as simple as knowing how to AVOID mixing mud!
Most clayers find that they need to have a system of keeping track of color successes and failures. This is especially important if you work with more than one brand of clay, and/or if the clay shifts color during baking. There are many ways to go about doing this, so do what works for you. Most systems, though, involve writing down the resulting formula, or at least noting the colors that were used if not their proportions. Some artists keep baked samples glued onto index cards in a file box or pocket photo album. Some artists like to keep bead strands of their samples. It just depends on what works for you, and perhaps how portable you need it to be.
I'd love to hear your comments on how you approach transforming package colors into custom colors and using them in your work. It's always nice to see how many tricks and variations people come up with.