The photoelectric total magnitudes and color indices published in the Third Reference Catalogue of Bright Galaxies (RC3, Cat. <VII/155>) are based on an analysis of ~26000 B, 25000 B-V, and 17000 U-B multiaperture measurements available up to mid 1987 from nearly 350 sources. This paper provides the full details of the analysis and estimates of internal and external errors in the parameters. The derivation of the parameters is based on techniques described by de Vaucouleurs & Corwin (1977ApJS...33..219D) whereby photoelectric multiaperture data are fitted by mean Hubble-type-dependent curves which describe the integral of the B-band flux and the typical B-V and U-B integrated color gradients. A sophisticated analysis of the residuals of these measurements from the curves was made to allow for the random and systematic errors that affect such data. The result is a homogeneous set of total magnitudes B^A^_T_, total colors (B-V)_T_ and (U-B)_T_, and effective colors (B-V)_e_ and (U-B)_e_ for more than 3000 bright galaxies in RC3.