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Colors of Type Ia supernovae in the first few days after explosion provide apotential discriminant between different models. In this paper, we present$g-r$ colors of 65 Type Ia supernovae discovered within 5 days from first lightby the Zwicky Transient Facility in 2018, a sample that is about three timeslarger than that in the literature. We find that $g-r$ colors are intrinsicallyrather homogeneous at early phases, with about half of the dispersionattributable to photometric uncertainties($\sigma_\mathrm{noise}\sim\sigma_\mathrm{int}\sim$ 0.18 mag). Colors arenearly constant starting from 6 days after first light ($g-r\sim-0.15$ mag),while the time evolution at earlier epochs is characterized by a continuousrange of slopes, from events rapidly transitioning from redder to bluer colors(slope of $\sim-0.25$ mag day$^{-1}$) to events with a flatter evolution. Thecontinuum in the slope distribution is in good agreement both with modelsrequiring some amount of $^{56}$Ni mixed in the outermost regions of the ejectaand with "double-detonation" models having thin helium layers($M_\mathrm{He}=0.01\,M_\odot$) and varying carbon-oxygen core masses. At thesame time, six events show evidence for a distinctive "red bump" signaturepredicted by "double-detonation" models with larger helium masses. We finallyidentify a significant correlation between the early-time $g-r$ slopes andsupernova brightness, with brighter events associated to flatter colorevolution (p-value=0.006). The distribution of slopes, however, is consistentwith being drawn from a single population, with no evidence for two componentsas claimed in the literature based on $B-V$ colors.

ZTF Early Observations of Type Ia Supernovae III: Early-Time Colors as a Test for Explosion Models and Multiple Populations