  We compute radial color gradients produced by an outwardly propagating
circular wave of star formation and compare our results with color gradients
observed in the classical ring galaxy, the ``Cartwheel''. We invoke two
independent models of star formation in the ring galaxies. The first one is the
conventional density wave scenario, in which an intruder galaxy creates a
radially propagating density wave accompanied by an enhanced star formation
following the Schmidt law. The second scenario is a pure self-propagating star
formation model, in which the intruder only sets off the first burst of stars
at the point of impact. Both models give essentially the same results.
Systematic reddening of B-V, V-K colors towards the center, such as that
observed in the Cartwheel, can be obtained only if the abundance of heavy
elements in the star-forming gas is a few times below solar. The B-V and V-K
color gradients observed in the Cartwheel can be explained as a result of
mixing of stellar populations born in a star-forming wave propagating through a
low-metallicity gaseous disk, and a pre-existing stellar disk of the size of
the gaseous disk with color properties typical to those observed in nearby disk
galaxies.
