  The best natural candidates for the realization of color superconductivity
are quark stars -not yet confirmed by observation- and the extremely dense
cores of compact stars, many of which have very large magnetic fields. To
reliably predict astrophysical signatures of color superconductivity, a better
understanding of the role of the star's magnetic field in the color
superconducting phase that realizes in the core is required. This paper is an
initial step in that direction. The field scales at which the different
magnetic phases of a color superconductor with three quark flavors can be
realized are investigated. Coming from weak to strong fields, the system
undergoes first a symmetry transmutation from a Color-Flavor-Locked (CFL) phase
to a Magnetic-CFL (MCFL) phase, and then a phase transition from the MCFL phase
to the Paramagnetic-CFL (PCFL) phase. The low-energy effective theory for the
excitations of the diquark condensate in the presence of a magnetic field is
derived using a covariant representation that takes into account all the
Lorentz structures contributing at low energy. The field-induced masses of the
charged mesons and the threshold field at which the CFL $\to$ MCFL symmetry
transmutation occurs are obtained in the framework of this low-energy effective
theory. The relevance of the different magnetic phases for the physics of
compact stars is discussed.
