  We present space-based ultraviolet/optical photometry and spectroscopy with
the Swift Ultra-Violet/Optical Telescope and Hubble Space Telescope,
respectively, along with ground-based optical photometry and spectroscopy and
near-infrared spectroscopy of supernova SN2017erp. The optical light curves and
spectra are consistent with a normal Type Ia supernova (SN Ia). Compared to
previous photometric samples in the near-ultraviolet (NUV), SN2017erp has
colors similar to the NUV-red category after correcting for Milky Way and host
dust reddening. We find the difference between SN2017erp and the NUV-blue
SN2011fe is not consistent with dust reddening alone but is similar to the SALT
color law, derived from rest-frame UV photometry of higher redshift SNe Ia.
This chromatic difference is dominated by the intrinsic differences in the UV
and only a small contribution from the expected dust reddening. Differentiating
the two can have important consequences for determining cosmological distances
with rest-frame UV photometry. This spectroscopic series is important for
analyzing SNe Ia with intrinsically redder NUV colors. We also show model
comparisons suggesting that metallicity could be the physical difference
between NUV-blue and NUV-red SNe Ia, with emission peaks from reverse
fluorescence near 3000 Angstroms implying a factor of ten higher metallicity in
the upper layers of SN2017erp compared to SN~2011fe. Metallicity estimates are
very model dependent however, and there are multiple effects in the UV. Further
models and UV spectra of SNe Ia are needed to explore the diversity of SNe Ia
which show seemingly independent differences in the near-UV peaks and mid-UV
flux levels.
