A multi-color LED illumination device and specifically a lens comprising a cylindrical opening extending into the lens from a light entry region at which one or more LEDs are configured. A concave spherical surface extends across the entirety of the light exit region of the lens, and a TIR outer surface shaped as a CPC extends between the light entry region and the light exit region. There are various diffusion surfaces placed on the sidewall surface of the cylindrical opening, as well as its upper planar surface and the exit surface of the lens. Lunes can also be configured on the sidewall surfaces of the cylindrical opening. The combination of lunes, diffusion elements, and the overall configuration of the lens provides improved color mixing and output brightness using three interactions in a first portion of light and two interactions in a second portion of light. Those interactions includes two refractions either with an intermediate reflection or not, all of which are necessary to achieve the improved performance of the multi-color LED illumination device and lens hereof.