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While the width-luminosity relation (WLR) among type Ia supernovae (slower isbrighter) is one of the best studied properties of this type of events, itsphysical basis has not been identified convincingly. The 'luminosity' is knownto be related to a clear physical quantity -- the amount of $^{56}$Nisynthesized, but the 'width' has not been quantitatively linked yet to aphysical time scale. We show that the recombination time of $^{56}$Fe and$^{56}$Co from doubly to singly ionized states causes the typical observedbreak in the color curve B-V due to a cliff in the mean opacities, and is arobust width measure of the light curve, which is insensitive to radiationtransfer uncertainties. A simple photospheric model is shown to predict therecombination time to an accuracy of $\sim5$ days, allowing a quantitativeunderstanding of the color WLR. Two physical times scales of the widthluminosity relation are shown to be set by two column densities -- the totalcolumn density which sets the gamma-ray escape time $t_0$ (previous Paper I)and the $^{56}$Ni column density which sets the recombination time (this PaperII). Central detonations of sub-$\rm M_{ch}$ WDs and direct WD collision modelshave gamma-ray escape times and recombination times which are consistent withobservations across the luminosity range of type Ia's. Delayed detonationChandrasekhar mass models have recombination times that are broadly consistentwith observations, with tension at the bright end of the luminosity range andinconsistent gamma-ray escape times at the faint end.

Type Ia supernovae have two physical width-luminosity relations and they favor sub-Chandrasekhar and direct collision models - II. Color evolution