The Art of Logo.
Contrast is an immensely powerful concept, probably the most powerful among the design tools.
Once you start analyzing, you will be surprised by the number of compositions where contrast is responsible for the overall arresting impression. At first sight, contrast may seem to you a pretty simple, probably even primitive, concept. One of the most important design skills is to choose the level of contrast that's necessary for your case. When it comes to practice, this recommendation reveals what may be considered a paradox. For instance, the two squares on Fig. 5 have contrasting colors, but their shape and size are the same.