Color wheel
A color wheel shows the visible spectrum of colors marked out in a circle. You rotate the top layer of the color wheel, which highlights the relationship between the colors. You can discover the complementary or harmonizing color of a chosen color by doing so.

Color spectrum in watercolor
To understand the color spectrum, you could try recreating it. Here I used watercolor. You can also buy kits, such as Create-a-Color-Wheel, as mentioned above under "Where can I get a color wheel?".

Primary
Red, yellow and blue are the main colors on the color spectrum from which other colors are created. These are known as primary colors.

Secondary
Secondary colors, such as green, are created from primary colors, such as yellow and blue.

Tertiary
Tertiary colors are created from secondary colors. In my experience, however, in terms of paint the more colors you mix together the muddier the end result can be. I tend to mix from two colors and at the most three. If you want to match a color, it can sometimes be easier if you buy a paint that is closest to that color.

Red and green, complementary colors
Red and green are complementary colors as they appear on opposite sides of the spectrum.

Harmonizing colors
Oranges and reds are harmonizing colors as they appear close together on the color wheel.

Triad
The easiest way to balance color is to use triads. These are colors that are equidistant on the wheel, or in other words three colors that are complementary to each other. For example, orange, green and purple form a triad of colors. If you use this combination in a painting, the overall effect should look balanced.

Combinations
Harmonizing colors, say reds and oranges, can be balanced against a complementary color, which in this instance would be green.

Alternative to using black
If you add yellow to purple, it produces a range of colors that are less vibrant and darkened by the addition. A great alternative to using black.

Alternative to using black
If you add green to red, it will produce a range of colors suitable for adding shade and darker tones to your paintings.

Red, a prominent color
Something to note about red is that it tends to be a prominent color. Blues and greens recede more into the background. If you create a painting that has red in it, the red will stand out more than the other colors. Traditionally, blues and greens have been used to suggest distance in paintings by using them near the horizon line. Prominent colors can be used to create a focus in an image.

Accents
Accent colors are bright colors that are offset against softer, neutral colors, like creams or grays.

Pastels
Pastel colors are created by adding white or by adding water. Watercolor purists advocate using the whiteness of the paper and the translucency of the paint to indicate lighter tones, rather than adding white. I often find that this is too difficult to achieve and end up using white. It is a good idea when using watercolors to try them out on a scrap of paper, before using them in your painting. Because of their transparent nature, watercolors look different on paper (when the white of the paper shows through). Acrylics however, are more opaque unless a lot of water is added.