Transition metal trichlorides show peculiar and versatile magnetic properties. Whereas CrCl3 is a layered antiferromagnet with potential applications as an ultrathin two-dimensional magnet, α-RuCl3 may host a spin-liquid state driven by Kitaev interactions. The interest to control their material properties by chemical modifications is immense, both from an application related and from a fundamental point of view. Here, by studying CrCl3, Cr0.5Ru0.5Cl3, and α-RuCl3 by photoemission and electron energy-loss spectroscopy, we find that transition metal substitution changes the optical properties of the host without compromising its underlying electronic structure. It does so by a Cr–Ru related charge transfer process across the Mott gap effectively opening up a new absorption channel below the principal gap edge of CrCl3. The Cr and Ru valencies as well as the respective valence band density of states remain stable for the mixed Cr0.5Ru0.5Cl3 compound. Our study underlines the potential of transition metal substitution as a means of material engineering of trichlorides.