  We report the experimental observation of strong two-color optical
nonlinearity in an ultracold gas of $^{85}\mathrm{Rb}$-$^{87}\mathrm{Rb}$ atom
mixture. By simultaneously coupling two probe transitions of $^{85}$Rb and
$^{87}$Rb atoms to Rydberg states in electromagnetically induced transparency
(EIT) configurations, we observe significant suppression of the transparency
resonance for one probe field when the second probe field is detuned at
$\sim1~\mathrm{GHz}$ and hitting the EIT resonance of the other isotope. Such a
cross-absorption modulation to the beam propagation dynamics can be described
by two coupled nonlinear wave equations we develope. We further demonstrate
that the two-color optical nonlinearity can be tuned by varying the density
ratio of different atomic isotopes, which highlights its potential for
exploring strongly interacting multi-component fluids of light.
