  The variability of the continuum spectral energy distribution has been
analyzed for a complete magnitude-limited sample of quasars in Selected Area
57, observed at two epochs in the photographic $U$, $B_J$, $F$ and $N$ bands
with the Mayall 4m telescope at Kitt Peak. Changes $\delta \alpha$ of the
spectral slope $\alpha$ appear correlated with brightness variations $\delta
log f_{\nu}$ indicating an average hardening of the spectrum in the bright
phases. This confirms that the correlation of variability with redshift, found
in a single observing band, is due to intrinsic spectral changes. The average
observed $\delta \alpha - \delta log f_{\nu}$ relation is consistent with the
spectral change due to temperature variation of a black body of about $2.5
\cdot 10^4 K$.
