  Galaxies over four decades in stellar mass are shown to obey a strong
correlation between u-K colors and atomic-gas-to-stellar mass ratios (G/S),
using stellar mass-to-light ratios derived from optical colors. The correlation
holds for G/S ranging from nearly 10:1 to 1:100 for a sample obtained by
merging the SDSS DR2, 2MASS, and HYPERLEDA HI catalogs. This result implies
that u-K colors can be calibrated to provide ``photometric gas fractions'' for
statistical applications. Here this technique is applied to a sample of ~35,000
SDSS-2MASS galaxies to examine the relationship of gas fractions to observed
bimodalities in galaxy properties as a function of color and stellar mass. The
recently identified transition in galaxy properties at stellar masses
\~2-3x10^10 Msun corresponds to a shift in gas richness, dividing low-mass
late-type galaxies with G/S ~ 1:1 from high-mass galaxies with
intermediate-to-low G/S. Early-type galaxies below the transition mass also
show elevated G/S, consistent with formation scenarios involving mergers of
low-mass gas-rich systems and/or cold-mode gas accretion.
