Art School Online is a great source offering 16 lessons in the theory of color. Here you’ll find the history of color, the meanings and Psychology of color, what is color perspective, the importance of value, color efects, manufactured colors, how to implement optical effects in design, and how to expressing yourself in color.

An Introduction to Using Patterns in Web Design – a great tutorial by Ryan Singer. Here you’ll find out how to use patterns in order to achieve better design decisions.

Color Harmony – an article that discloses the secrets of Web Color.

Design Psychology is an article written by Andy Rutledge – an interactive designer, writer and composer in Dallas, Texas.

Worqx.com is a great theoretical resource for Web designers. You can find here such worthy articles as: Color Theory Overview, Color Basics, Color Systems, Color Wheel, Complementary Colors, Color Combinations, Color & Contrast, Proportion and Intensity, Shades and Tints, Color Studies, and more.

What Is Color? is a course consisting of 8 parts: Introduction, Additive and Subtractive Color, Color Gamut and Describing Color, Color Wheels Page, Color Categories, Hue, Saturation, and Value Color, HSV, and RGB Models, CMYK Model.

Color Matters is an excellent educational resource where you can find useful articles on the color theory.

Color Theory – a short course consisting from four parts: Introduction, The Wheel, Other Terms and Types of Colors.

Color, Contrast and Dimension by Pegie Stark Adam is an online guide that explains color theory and shows how to use it in design through examples and exercises. The tour is split into several sections the power of color, the philosophy and theory of color, Color, Contrast & Dimension, Color on the Web, Eye-trac research.

Color Codes Matching Chart HTML The Pantone Matching System (PMS) allows users to mix percentages of base inks (like CMYK) to create new colors, either physically (spot colors) or on the printed document using screens to allow certain amounts of ink through and then overlaying the base colors.