Provided is an optical diffuser apparatus, for use adjacent a light engine in a lighting fixture for producing desired color-mixed light output. The apparatus includes a first diffuser panel and a second diffuser panel connected to the first diffuser panel. The first diffuser panel and the second diffuser panel are configured and arranged so that, when the apparatus is positioned for operation within the fixture, light emitted by the light engine at greater than a pre-determined threshold angle with respect to a centerline of the engine will pass through both the first diffuser panel and the second diffuser panel, thereby mixing colors of the light toward the desired color-mixed light output.