Now, I could make an argument that contrast covers just about everything, but I thought I'd include balance because, while contrast provides balance, balance is important enough as a separate entity. Contrast applies to size, color, positioning, and typography. Again, this applies to every element of design. Most importantly, contrast provides emphasis, which gives the audience a visual clue to what's really important. Balance From here, the combination of your contrasting points lead to balance.
Balance depends primarily on the mass and tension created by individual elements.
Balance contributes to and is affected by visual flow, too. Your good reason could be, for example, designing for WebTV, where cold on warm actually increases readability. And, the most vivid combinations are going to be between your contrasting colors, with the warm on bottom and cold on top. Of course, you must also consider the shades and saturation of your colors when you experiment with color combinations. The other biggie for readability is handling of text columns.