  Colors of Type Ia supernovae in the first few days after explosion provide a
potential discriminant between different models. In this paper, we present
$g-r$ colors of 65 Type Ia supernovae discovered within 5 days 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
($\sigma_\mathrm{noise}\sim\sigma_\mathrm{int}\sim$ 0.18 mag). Colors are
nearly 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 continuous
range 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. 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
($M_\mathrm{He}=0.01\,M_\odot$) and varying carbon-oxygen core masses. At the
same time, six events show evidence for a distinctive "red bump" signature
predicted by "double-detonation" models with larger helium masses. We finally
identify a significant correlation between the early-time $g-r$ slopes and
supernova brightness, with brighter events associated to flatter color
evolution (p-value=0.006). The distribution of slopes, however, is consistent
with being drawn from a single population, with no evidence for two components
as claimed in the literature based on $B-V$ colors.
