The color quality of overhead projection transparency is affected by the transparency print, the projector, the projection screen, and the viewing environment. For color calibration, there are commercial measurement instruments capable of transparency sample spectral and color transmittance measurement. However, projection stray light, sample scattering as in samples printed with EP technologies, and projector design parallax limitation as in reflective-type projectors makes using measured transmittance data to represent the projected colors inadequate. In this paper, we report our effort and findings on overhead projection transparency color measurement and calibration. We devised an automated measurement system based on a spectroradiometer to measure multiple projected colors automatically; the results are compared with that obtained by direct sample transmittance measurement with both diffuse illumination and collimated illumination. We concluded that projected colors should be measured for superior overhead projection transparency color calibration to compensate for regular stray light, sample scattering, and projector optical design limitations.