  Gas-giant planets that form via core accretion might have very different
characteristics from those that form via disk-instability. Disk-instability
objects are typically thought to have higher entropies, larger radii, and
(generally) higher effective temperatures than core-accretion objects. We
provide a large set of models exploring the observational consequences of
high-entropy (hot) and low-entropy (cold) initial conditions, in the hope that
this will ultimately help to distinguish between different physical mechanisms
of planet formation. However, the exact entropies and radii of newly-formed
planets due to these two modes of formation cannot, at present, be precisely
predicted. We introduce a broad range of "Warm Start" gas-giant planet models.
Between the hottest and the coldest models that we consider, differences in
radii, temperatures, luminosities, and spectra persist for only a few million
to a few tens of millions of years for planets that are a few times Jupiter's
mass or less. For planets that are ~five times Jupiter's mass or more,
significant differences between hottest-start and coldest-start models persist
for on the order of 100 Myrs. We find that out of the standard infrared bands
(J, H, K, L', M, N) the K and H bands are the most diagnostic of the initial
conditions. A hottest-start model can be from ~4.5 magnitudes brighter (at
Jupiter's mass) to ~9 magnitudes brighter (at ten times Jupiter's mass) than a
coldest-start model in the first few million years. In more massive objects,
these large differences in luminosity and spectrum persist for much longer than
in less massive objects. We consider the influence of atmospheric conditions on
spectra, and find that the presence or absence of clouds, and the metallicity
of an atmosphere, can affect an object's apparent brightness in different bands
by up to several magnitudes.
