Correlation of the H I-to-stellar mass ratio (M_HI/M_*) with g − i (left) and NUV − r (middle) for the low-z calibrating sample. The best-fit models are plotted as solid red lines (see the text for a detailed description). Right-hand panel: relation between g − i and NUV − r for the same set of galaxies. The low-z calibrating sample is plotted as cyan dots, the RESOLVE sample is plotted as magenta crosses, and the final ALFALFA 100% catalogue (α.100) is shown as black-and-white contours in each panel.