We address the problem of calibrating the colors of a multiple projector tiled display. Differences in the characteristics of the projectors lead to varying color and intensity among the tiles of the display. These differences do generally not only arise between two different projectors, but also between different spatial locations on the projected surface of one single projector. In this paper, we present two algorithms for calibrating the colors of a tiled display. The first method is an automatic procedure to make the luminance uniform over the projected surface of one single beamer, using a common digital camera as a luminance measuring device. The second method aims at uniformizing the colors of multiple projectors by computing the set of color commonly displayable by all the beamers (the common gamut). We show how to find an optimal color gamut displayable by all the projectors in linear time, and derive the color conversion from one color gamut to the common gamut. We successfully tested the method on a tiled display made of 48 projectors with large chrominance shifts.