Abstract: An optical device comprises a pair of contrastingly-colored sources of light, means for adjusting the brightness of the sources of light relative to each other, and a mixing element formed with a light-entrance portion to permit entrance into the mixing element of light emanating from the sources and with a display surface that communicates optically with the entrance portion and facilitates viewing of the light entering the mixing element through the entrance portion and propagated through the mixing element to the display surface, the light as displayed having a color that depends on the relative brightness of the sources. The mixing element has at least in part generally the shape of a cylinder formed with an axial bore so that the mixing element has an outer surface and an inner surface.