Carl Dair warns us to give careful thought to which elements needs to be emphasized, and to pay attention to the tonal values of the colors used. Color alters the scale of relationship between similar letterforms. A typographic color change allows the designer to highlight structure and invigorate a page. Rich spatial perceptions can be achieved through varing colors and even just varying color intensities and this allows the designer to seprate information as well as creating visual interest.
Take a look at the quote below:
Notice how in the quotation, color is used to emphasize priority words, at different opacities depending on their importance.