Red and blue color purities and transmittances of transmissive display by MEMS (micro electro mechanical system) etalon (Fabry-Perot interferometer) have been greatly improved by changing reflecting electrode materials. 22% and 63% relative color purity deviation (CPD) reduction, and 27% and 255% transmittance improvement were achieved on red and blue pixels, respectively. The definition of relative CPD also provided a simple and fast way to judge two colors' purities from a target point on CIE 1931 Chromaticity Diagram without measurement of their luminance.