Updated answer 01/23/2017:
Using light to control the 'fades'/volume of each channel by using a LED and photoresistor(s). You can simply control the LED with an Arduino. Because of the LED is driven by PWM, you need to change the PWM frequency. See also example at my answer (and the reason why).
I made some drafts/examples at my question, my question is about if there is component that combines the two. There is, called vectrols but they are very expensive. You can make your own, it's cheap! More info and pictures at my question and answer.
Changed and simplified schematic: