We show that trivalent manganese, Mn3+, imparts an intense blue color to oxides when it is introduced at dilution in trigonal bipyramidal, coordination. Our optical measurements and first-principles density functional theory calculations indicate that the blue color results from an intense absorption in the red/green region. This absorption is due in turn to a symmetry-allowed optical transition between the valence-band maximum, composed of Mn 3d(x)(-y)(,xy)(2)(2) states strongly hybridized with O 2p(x,y) states, and the narrow Mn 3d(z)(2)-based conduction-band minimum. We begin by demonstrating and explaining the effect using a well-defined prototype system: the hexagonal YMnO3-YInO3 solid solution. We then show that the behavior is a general feature of diluted Mn3+ in this coordination environment.