Use the left-hand toolbar of this site to investigate an online textbook for the language of design. It provides visual examples and explanations of line, form, color, color psychology, texture, balance, proportion and many more...

Starts with explaining visual elements, goes through design principles, balance, visual effects, and more. Teaching tips included too.

A site by watercolor artist John Lovett. He describes the Elements of Art and Principles of Design and gives tips and techniques for watercolor painting and for composition.