A transmission medium and protocol is provided for bi-directional communication between an audio system and a peripheral device. An estimate of the surface color is processed and a projection color is selected as a function of the surface color estimate and a chosen color so that the projection color yields the chosen color or a color substantially similar to the chosen color when projected onto the surface. When the LOA falls below the security threshold, the processor may be capable of generating the challenge, performing the given mobile device operation responsive to valid response data, and adding recent input data to the statistical behavioral model responsive to receipt of the valid response data.