The following is an excerpt about Visual Interface Design from the book About Face 3:
Regardless of how much effort you put into understanding your product’s users and crafting behaviors that help them achieve their goals, these efforts will fall short unless significant work is also dedicated to clearly communicating these behaviors to users in an appropriate manner. With interactive products, this communication commonly happens visually, through a display (although in some cases you must communicate product behavior through physical properties such as the shape or feel of a hardware button).
Visual interface design is a frequently misunderstood discipline, largely because of its similarities to visual art and graphic design. It is also commonly mischaracterized as “skinning” the interface; we’ve even heard people refer to it as “hitting the product with the pretty stick.”
In our practice, we’ve come to recognize that visual interface design is a critical and unique discipline, and it must be conducted in concert with interaction design and industrial design. It has great power to influence the effectiveness and appeal of a product, but for this potential to be fully realized, visual design must not be an afterthought (i.e.,“putting lipstick on a pig”), but should be thought of as an essential tool for satisfying user and business needs.
Visual interface design requires several related skills, depending on the product in question. To create an effective and engaging user interface, a designer must have a command of the basic visual properties — color, typography, form, and composition— and must know how they can be used to effectively convey behavior and information and create a mood or visceral response. Interface designers also need a fundamental understanding of the interaction principles and interface idioms that shape the behavior of the product.