Many alterations to the appearance of paintings may be attributed to color change in nonoriginal layers, most notably varnishes; the cleaning and restoration of paintings frequently reveal such effects. Color changes due to the alteration of original pigments are less obvious, often being apparent only after an examination of the paint layer structure, and are seldom reversible. Examples of a number of types of color change are given, along with an introduction to the techniques available for detecting color change. To quantify color change, it is first necessary to make accurate color measurements of an area before and after such a change occurs. A number of the methods available for measuring the surface color of a painting are discussed. The way in which the difference between before and after measurements may be converted into a meaningful expression of color difference are explained. The article also explores the factors which are responsible for color change and outlines the steps which museums and art galleries might take to prevent future deterioration of colored pigments. -- AATA