The presence of a varnish layer modifies the light scattered and reflected by a painting. A general equation, valid for any bidirectional configuration, allows the expression between the final and initial diffuse reflectance by taking into account the influence of the different interfaces. This equation can be simplified to a homothetic relationship. In this case, the final colorimetric coordinates are directly expressed in terms of the initial coordinates. The results are then compared with the exact computations. Hue is not affected by the addition of a varnish. Lightness and chroma variations depend on the refractive index of the paint (n(p)) to that of the varnish (n). If n(p) > n, lightness and chroma increase. Conversely, if n(p) > n, lightness always decreases, but chroma increases for light initial painting and chroma decreases for dark ones. (C) 2004 Wiley Periodicals, Inc.