hroughout history, bright colours have contributed to the advancement of art and science through inspiring creativity, improving mental wellbeing, and enhancing the transfer of information. However, since many common pigments are produced using toxic materials, there has been a recent burst of interest in producing non-toxic, colourfast, long-lasting pigments for various colour treatment applications [1], led by the discovery of a durable, environmentally friendly inorganic blue pigment Y(Mn_l-xln_x)O3 [2]. The two end compounds of this mixture, YlnO3 and YMnO3, are white and black, respectively. Both compounds crystallize in a layered hexagonal lattice, while the intermediate compounds form a solid solution in which In and Mn occupy the same lattice sites. Mixing a small amount of Mn into the YlnO3 lattice results in a startling blue colour originating from absorption of other colours by electronic transitions from the Mn 3d states, including nominally forbidden d-d transitions. These transitions are made possible by the non-centrosymmetric trigonal biypramidal (TBP) environment of 0 around the Mn ions [2,3]. In this study, we present a thorough characterization of the structural and spectral characteristics of Y(Mn_1-xln_x)O3 across the full compositional range. We discuss the origin of the strong blue colour and its correlation with the crystallographic properties of this oxide material.