  The early, highly time-variable X-ray emission immediately following GRBs
exhibits strong spectral variations that are unlike the temporally smoother
emission which dominates after $t\sim 10^3$ s. The ratio of hard channel
(1.3-10.0 keV) to soft channel (0.3-1.3 keV) counts in the Swift X-ray
telescope provides a new measure delineating the end time of this emission. We
define $T_{H}$ as the time at which this transition takes place and measure for
59 events a range of transition times that span $10^2$ s to $10^{4}$ s, on
average 5 times longer than the prompt $T_{90}$ duration observed in the
Gamma-ray band. It is very likely that the mysterious light curve plateau phase
and the later powerlaw temporal evolution, both of which typically occur at
times greater than $T_{H}$ and hence exhibit very little hardness ratio
evolution, are both produced by external shocking of the surrounding medium and
not by the internal shocks thought responsible for the earlier emission. We use
the apparent lack of spectral evolution to discriminate against proposed models
for the plateau phase emission. We favor energy injection scenarios with a
roughly linearly increasing input energy versus time for six well sampled
events with nearly flat light curves at $t\approx 10^3-10^4$s. Also, using the
transition time $T_{H}$ as the delineation between the GRB and afterglow
emission, we calculate that the kinetic energy in the afterglow shock is
typically a factor of 10 lower than that released in the GRB. Lack of hardness
variations in these three events may be due to a very highly relatavistic
outflow or due to a very dense circumburst medium. There are a handful of rare
cases of very late time $t>10^4$s hardness evolution, which may point to
residual central engine activity at very late time.
