The colours of the solutions containing Pt Chini clusters depend on their size (or nucleation number). The theoretical UV-Vis absorption spectra for the Chini clusters (n=1-8) were calculated -see Figure for the simulated spectra. Two distinct absorption peaks in the visible range of the spectrum are found. To assign the character of the electronic transitions responsible for such a spectrum, the character of the calculated transitions was analysed for the Chini cluster with n=3 -Figures S3 and S4 in SI. The broad, red-shifted band (lower energy peak) corresponds to inter-triangle transitions. They occur between diferent Pt3 units, involving interactions across the cluster. The narrow, higher energy band is associated with intra-triangle transitions, where electronic excitations involve CO orbitals within the same Pt3 unit. These transitions are more localized, higher-energy, and afect the transverse direction of the cluster.