The principles of design are the thinking behind what you put on the paper

Emphasis (contrast) is a principle of design. 

• happens when two elements are different
• greater the difference = greater contrast
• different colour, value, size and type
• opposites such as colour, light/dark, direction up/down

 Meanings can be attached to emphasis by contrast

• used to emphasise 
• used to highlight
• used to catch your eye
• adds interest
• shows what is important
• can be too much if you overdo it

Learn more here at google site (has a quiz too)