ZTF early observations of type Ia supernovae. III.

In this paper, we present g-r colors of 65 SNe Ia discovered within 5d from first light by the Zwicky Transient Facility in 2018, a sample that is about three times larger than that in the literature. We find that g-r colors are intrinsically rather homogeneous at early phases, with about half of the dispersion attributable to photometric uncertainties ({sigma}_noise_~{sigma}_int_~0.18mag).

Colors are nearly constant starting from 6 days after first light (g-r~-0.15mag), while the time evolution at earlier epochs is characterized by a continuous range of slopes, from events rapidly transitioning from redder to bluer colors (slope of ~-0.25mag/day) to events with a flatter evolution. The continuum in the slope distribution is in good agreement both with models requiring some amount of ^56^Ni mixed in the outermost regions of the ejecta and with "double-detonation" models having thin helium layers (M_He_=0.01M_{sun}_) and varying carbon-oxygen core masses.