The Color-magnitude Relation of Disk Galaxies

15257 face-on disk galaxies are selected from the second group of data released by the Sloan Digital Sky Survey, and their color-magnitude relation is studied statistically. It is shown that the three colors g − r, r − i and r − z are each closely correlated with the r-band absolute magnitude, i.e., the brighter the galaxies, the redder the colors, and the smaller the dispersion of color indexes. The implications of the observed color-magnitude relations on the star formation history of disk galaxies are briefly discussed.