  We study a Nambu-Jona-Lasinio (NJL) type model with two-flavor quark matter
in $\beta$-equilibrium. It turns out that the system develops instabilities in
the dispersion relations of the diquark fields, i.e., the velocity squared
$v^2$ becomes negative in a certain region of the electron chemical potential.
The critical point is the same as that of the chromomagnetic instability. The
results imply the existence of spatially inhomogeneous diquark condensates in
the genuine vacuum. We also discuss gauge equivalent classes between the
inhomogeneous diquark condensates and gluon condensates.
