A motor-generator system of a hybrid drive with an internal combustion engine and an electric machine connected to an energy storage device via a converter device. If the colored light source is driven by the preset duty cycle, the method includes modifying the reference data based on the detected light from the colored light source and adjusting the amount of colored light by controlling a driving signal to the colored light sources based on the modified reference data. A work unit that requires processing by a speech engine can be detected for the call.