  We identified a general course of classical nova outbursts in the B-V versus
U-B color-color diagram. It is reported that novae show spectra similar to
those of A--F supergiants near optical light maximum. However, they do not
follow the supergiant sequence in the color-color diagram, neither the
blackbody nor the main-sequence sequence. Instead, we found that novae evolve
along a new sequence in the pre-maximum and near-maximum phases, which we call
"the nova-giant sequence." This sequence is parallel to but \Delta (U-B)
\approx -0.2 mag bluer than the supergiant sequence. This is because the mass
of a nova envelope is much (\sim10^{-4} times) less than that of a normal
supergiant. After optical maximum, its color quickly evolves back blueward
along the same nova-giant sequence and reaches the point of free-free emission
(B-V=-0.03, U-B=-0.97), which coincides with the intersection of the blackbody
sequence and the nova-giant sequence, and remains there for a while. Then the
color evolves leftward (blueward in B-V but almost constant in U-B), owing
mainly to the development of strong emission lines. This is the general course
of nova outbursts in the color-color diagram, which was deduced from eight
well-observed novae in various speed classes. For a nova with unknown
extinction, we can determine a reliable value of the color excess by matching
the observed track of the target nova with this general course. This is a new
and convenient method for obtaining the color excesses of classical novae.
Using this method, we redetermined the color excesses of twenty well-observed
novae. The obtained color excesses are in reasonable agreement with the
previous results, which in turn supports the idea of our general track of nova
outbursts.
