  We investigate the effects of an external magnetic field in the gluon
dynamics of a color superconductor with three massless quark flavors. In the
framework of gluon mean-field theory at asymptotic densities, we show that the
long-range component $\widetilde{H}$ of the external magnetic field that
penetrates the CFL phase produces an instability when its strength becomes
larger than the Meissner mass of the charged gluons. As a consequence, the
magnetic field causes the formation of a vortex state characterized by the
condensation of charged gluons and the creation of magnetic flux tubes. Inside
the flux tubes the magnetic field is stronger than the applied one. This
antiscreening effect is connected to the anomalous magnetic moment of the gluon
field. We suggest how this same mechanism could serve to remove the
chromomagnetic instabilities existing in gapless color superconductivity.
