Clear trend from orange (early type) to blue (late type) arising from systematically higher relative star formation rates in late type galaxies (From Roberts and Haynes 1994 ARAA 32 115).
Three galaxies spanning Early -- Intermediate -- Late Hubble types. These SDSS images nicely reveal the true colors of the galaxies, which clearly change from orange to blue along the Hubble sequence.
Current star formation rate relative to the long-term average, using an estimate of the disk mass and age (from Kennicutt 1983 ApJ 272 54)
Equivalent width of H
as a diagnostic of current star formation, increases to later types (From Kennicutt 1983 AJ 88 1094).