The final color quality of overhead projection transparencies depends on 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 transmittance and transmittance color measurement. However, projection stray light, sample scattering for samples printed with EP technologies, and projector design parallax limitation as in reflective type projectors makes using directly measured transmittance data to represent the projected colors inadequate. In this article, we report our effort and findings on overhead projection transparency color measurement. 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 for superior overhead projection transparency color calibration, projected colors should be measured to compensate for stray light, sample scattering, and projector optical design limitations.