A comparative study of photometric observations made using the Johnson VRI (1965) and Cousins RI (1980) color systems is presented. The R and I passbands of the different versions of the Johnson system used in published studies are compared, considering both S1-based and S20-GaAs-based bandpasses. Discrepancies are attributed to the Balmer lines, the Paschen discontinuity and reddening in hot stars, and luminosity and abundance-related metal-line and molecular-band absorption differences in late-type stars. Data obtained with the different Johnson systems are evaluated by comparing precise Cousins-system measurements of the same stars, and linear transformations are developed for converting and comparing data. The color indices of Argue (1967) and Jacobson (1970) are also treated. The results of spectral-response tests on two RCA 31034-02 GaAs tubes are presented in an appendix.