Make all visual distinctions as subtle as possible, but still clear and effective.
Edward R. Tufte—Visual Explanations p.73
Tufte describes this principal as the ‘Occam’s razor’ of information design. You take a visual element, such as a border on a image or a background for a table cell, then ask yourself: is the difference here noticeable, and no more.
Use less to show more. When creating designs, screens, web pages, information graphics, the idea is to use just noticeable differences.
This principal is all about contrast. Too much and things look and feel wrong—due to the additional noise; not enough and the desired effect is lost. With just enough difference you’ll emphasise what is important and deemphasise what is not.