Contrast
Now that we know how different colours can be combined, we need to talk about adding contrast to your colours. Contrast in colour theory works much the same way as you've learned about layout; with colour theory you are also concerned with Readability and Legibility.

Legibility is how easy it is to identify text in short phrases such as headlines, signs, slogans, etc.

Readability is how easy it is to read lots of text: sentences, paragraphs, and pages of text.

Along with typeface and size, colour contrast is the most important way to make sure your project is both readable and legible.

With regards to colour theory, contrast is the difference between two colours. On the screen the amount of contrast required varies with different parts of the page. You usually want a high contrast between a text and its background colour. But too high contrasts in the background itself might give an unsettled and messy impression. Black and white is the highest contrast possible. In print, black on white is the easiest to read; from far away, black on yellow (such as caution signs ) is most legible.

Colours can contrast in hue, value and saturation, but there are many different sort of contrasts defined by colour theorists throughout the years.

Lightness contrasts create a very distinct image. In hue contrasts, vivid tones have the most contrast, and any opposite colours along the hue circle are compatible. Contrasts in saturation are often used in a one-point colour scheme – use different saturations of the same colour to create a scheme that is easy to print. Saturation contrasts are more difficult than hue or value.

Simultaneous contrast
This is a contrast effect that is created by our eyes' tendency to require a complementary colour. You can get this effect by combining two bright colours that are not complementary, or to use a single bright colour against a grey background. This gives a feeling of instability and tension, and should be used with caution. These contrasts cause movement in the eye, and should never be used with text that must be read.

combination of contrasts
While the contrasts above can be efficient used one at the time, the most common is to use a combination of them, especially for text where you need a high contrast. The top picture to the left shows blue and its split complementary colour orange. This is a combination that has a high contrast of hue which gives a rather vibrant combination that can be tiring to the eyes. By changing the value and saturation as in the next picture you will get a combination which is much more pleasing to the eye, more readable, and still a split complementary combination.

Working against the natural values of the colours can often have bad effects. For instance, yellow is naturally a lighter colour than its complement blue. A combination where you have a yellow that is darker than the blue would feel strange.

Colours and text
As mentioned above, using the right contrast is especially important for text. Using the wrong colours can decrease the readability drastically, and it will quickly tire the eyes of the reader. Highest readability is black text on white background. Black and yellow is another combination which usually has a high readability, as does blue and white.

Green text on red, and red text on green are for many particularly hard to read. A combination of red and blue creates a vibrating effect that can make reading very difficult.

White on black produces high contrast, but it is more difficult to read because black is perceived as being heavier than white and, thus, squeezes it out a little.

The same sort of effect happens with warm and cool colors. The warm colors are perceived as advancing out of the screen slightly, and the cool colors recede. This means that a warm/dark color is best for text, while a cool/light color works best for the background.

In any case, a white, black, or a very light desaturated background always looks the most professional (in that order).

One last thing...
Another thing that can be good to keep in mind when making colour selections is colour blindness. Some 8% of the population has some form of colour blindness, and by choosing the wrong colours you can make your page virtually unreadable for them. There are some colours which are worse than others, for instance red and green. Blues and yellows are usually better to use for this reason, and you should make sure that there is a strong contrast between a text and its background. Also, make your design so that colour is not the only visual cue.