Isotope effects in two-photon two-color photoionization are investigated by a combined theoretical and experimental study of the ionization of xenon atoms. A combination of variable polarization synchrotron and laser radiations are used to excite the $5{{{\rm p}}^{5}}{{(}^{2}}{{{\rm P}}_{1/2}})4{\rm f}{{[\frac{5}{2}]}_{2}}$ autoionizing resonance via the intermediate $5{{{\rm p}}^{5}}{{(}^{2}}{{{\rm P}}_{3/2}})5{\rm d}{{[\frac{3}{2}]}_{1}}$ state. Electrons and ions are detected in coincidence in order to extract the photoelectron angular distributions and the values of the linear and circular dichroism and to determine how these depend on the isotope. A complete theoretical model of the two-photon process in atoms is given in order to describe these parameters as a function of the polarization of the exciting light sources (both linear and circular polarization). Furthermore, the hyperfine depolarization due to the coupling of the electronic and nuclear angular momenta in the intermediate state is taken into account. The results of the theoretical model are in agreement with the experimental results and allow estimation of the previously unknown hyperfine structure (HFS) constant for the case of overlapping HFS levels.