When I'm painting, I try to think about whether a color is transparent, semi-transparent, or opaque. Each pigment has different qualities which I become familiar with as I work with them, but then there's the broader concept of transparency to consider as well. Opaque paints will cover another layer completely. Transparent paints can be used as a glaze, sort of like putting colored glass over something solid - it changes the color some, but the other color still shows through it, thus making a more complex, interesting color. It's a powerful skill to have to understand how to use glazes to build up pure complex color or when to use opaques. I do not claim to have that skill to any extent yet, but I'm looking forward to increasing my skill in that area.
To help me, I've made a chart of which pigments are opaque, semi-transparent, or transparent to help me know what to use when. It was challenging to find all the info, but I got some of it from Gamblin's website (they're a wonderful company which makes great paints from great materials. I trust their stuff implicitly and count on them for good information) and from a classic guide to all things painterly, The Painter's Craft, by Ralph Mayer. He describes the pigments in detail and explains what they are best used for, where they are derived from, etc.
What I learned about materials is that all paints and pastels and oil sticks, etc. start from the same thing: pigment. Pigments come from the earth - literally - some of them are made from rocks or stones or dirt, from bugs, from cows' urine, from plants, or are manufactured by man more recently. Some are organic. Others are inorganic. I can buy yellow ochre, for instance, in powered form and use it to make any number of media. For example, if I add some binder in the form of liquid methyl cellulose (library paste, sort of), and some water, and perhaps a little bit of calcium carbonate (chalk), to pigment, I can then mash all that up, then roll it, and - voila! - magic! - I have a pastel after a day of drying time. Or I can add linseed oil and grind it with a muller on a piece of marble (think Girl with a Pearl Earring) and, after a while, I have oil paint. Each medium has it's own vehicle - chalk, oil, etc - which makes it and its properties unique, but all start from pigments. That's why it's helpful to understand the temperments of each pigment.
I tried to paste the table of pigments into this blog, but the formatting wasn't possible. Instead I've put it on my website, SusanSinger.com. While you're there, take a look around if you're interested! I have lots of paintings and other cool stuff there!