ArtLex
Art Lex: Value
Artlex's definition of value and how it applies to black and white and color in art. There are links to other sites to further explain the term and also related terms to explore.
Hanover College
Hanover College: Use of Visual Information in Art
This site from Hanover College gives a tutorial on issues of visual representation, such as depth, perspective, texture, shadow, illusion. Begins at an introductory level. Numerous pictures.
Other
Guidance/the Alphabet of Art/value
An informative site that explains Value as an element of art using examples of Rembrandt's work. This is a good juxtaposition between the works "Night Watch," and "The Anatomy Lesson," by Rembrandt.
J. Paul Getty Trust
J. Paul Getty Museum: Drawings by Rembrandt and His Pupils
Compare and contrast drawings by Rembrandt with those of his students to learn about his characteristic visual traits, especially his approach to line and his use of light and shadow to create the illusion of three dimensions.