In order to talk precisely about color, we need some precise terms. In the most useful system of describing color, three parameters are needed to clearly describe any color. Those parameters are hue, value, and chroma.
HUE is where a color falls on the spectrum (or color wheel). For example, a color could be greenish-blue. There are an infinite variety of hues, but we can divide them into groupings that are easy to work with. One way is with three primaries and three secondaries. It's useful to use red, blue, and yellow as the three primaries. The secondaries are orange, violet, and green. Secondaries can be mixed from primaries, but primaries are not mixable. For example, you can mix orange from yellow and red, but you can't take two non-reds and mix a red. Functionally, most pigment colors are not "pure;" they have a bias toward one of the colors next to them. It's hard to find a simple red, but there are lots of violet-reds and orange-reds. Same goes with yellow greens, violet blues, and so on. A few pigments are pretty close to pure primaries.
VALUE is how light or dark a color is. You can have a violet blue that is dark, light, or somewhere in between. The eye pays more attention to value than it does to anything else. Therefore, in drawing or painting, value is the most important parameter to get right. Black pigment (or a mixed black) is the darkest value available. White is the lightest value available.
Because the real world has a much broader value range than paint or other media do, the artist often needs to make frequent judgments about how to represent a particular effect. For example, last night I was drawing a kneecap in graphite on white paper. There was a strong highlight on the kneecap that I wanted to render precisely. In order to get the highlight to look as noticeable as it is in real life, I needed to darken down the area around the kneecap to get the right contrast. In doing so, I made the area around it darker than I otherwise would have rendered it. I had to make a choice, within my limited value range, of whether to be realistic with the lights, or the highlights. I couldn't do both at the same time.
CHROMA is how intense the color is. Neon yellow is very high in chroma. Brown is a low chroma orange-yellow or yellow orange (there is no brown on the color wheel; it's a zone within the overall color space of yellow and orange). The majority of colors that you can buy in a tube are high in chroma. But most of the world is low in chroma. Look around you; how much of what you see is really high intensity color? If you want to paint realistically, you need to learn to mix more neutral colors. There are a couple of ways to do that: one is to mix with complimentary colors (those that are on the other side of the color wheel). Another is to mix with a neutral grey of the same value (note: black and white make a blue-gray, not a neutral gray). It takes a lot of practice to get good control over mixing of neutrals and near-neutrals.
Update: Lawrence Humphrey (in email) kindly pointed out that I misstated the number of secondaries. That's been corrected.