Left: Correlation between u-K color and gas-to-stellar mass ratio, using HyperLeda HI data and SDSS/2MASS photometry.
For more info, read the paper.
Right: Distribution of 35,000 SDSS-2MASS galaxies in color-stellar mass space. Galaxies are binned and given an elliptical/spiral symbol based on mean concentration index, which is color-coded to show mean "photometric" gas fraction using the scale from the left panel. A clear shift in gas richness is apparent near the threshold mass region marked by vertical lines.