Four-step sequence of galaxies of increasing (g − i)_o color is identified in the color-mass diagram of panel a) when four groups of different morphological types/gas-content/nuclear properties are considered. Panel a) corrected color (g − i)_o vs. stellar mass diagram of optically selected galaxies of all morphological types. Red symbols represent ETGs. Green symbols represent HI-poor LTGs not detected by ALFALFA, while HI-rich LTGs detected by ALFALFA are given in blue. Large black symbols represent PSB (also named k+a, i.e., with Balmer lines in absorption) galaxies identified by Gavazzi et al. (2010). They all lie at log   (M_∗/M_⊙) < 10. Panel b) a similar gradual trend in the color-mass diagram is found when galaxies are grouped in bins of increasing local density. This panel shows the corrected color (g − i)_o vs. stellar mass diagram of optically selected galaxies of all morphological types, in three bins of decreasing local density: red = log   (1 + δ_1,1000) > 1.35, green = 0.7 < log  (1 + δ_1,1000) < 1.04; blue = log   (1 + δ_1,1000) < 0. Panel c) the specific star formation rate (SSFR) vs. stellar mass diagram of galaxies covered by ALFALFA and followed-up by Hα3. Galaxies are color-coded according to the following criterion: red = ETGs; green = LTGs undetected by ALFALFA (HI-poor); blue = LTGs detected by ALFALFA (HI-rich). Unsurprisingly, the statistics are very poor among ETGs and for HI-poor LTGs.