Page 1: Introduction
Contrast is one of the most effective ways to add visual interest to your page-a striking interest that makes a reader want to look at the page-and to create an organizational hierarchy among different elements. The important rule to remember is that for contrast to be effective, it must be strong. Don't be a wimp.
Contrast is created when two elements are different. If the two elements are sort of different, but not really, then you don't have contrast, you have conflict. That's the key-Robin's Principle of Contrast states, "If two items are not exactly the same, then make them different. Really different."
Contrast can be created in many ways. You can contrast large type with small type; a graceful oldstyle font with a bold sans serif font; a thin line with a thick line; a cool color with a warm color; a smooth texture with a rough texture; a horizontal element (such as a long line of text) with a vertical element (such as a tall, narrow column of text); widely spaced lines with closely packed lines; a small graphic with a large graphic.
But don't be a wimp. You cannot contrast 12-point type with 14-point type. You cannot contrast a half-point rule with a one-point rule. You cannot contrast dark brown with black. Get serious.
Source: The Non-Designer's Design Book by Robin Williams