Maybe something like this http://www.ebay.co.uk/itm/Color-Sensor-Color-Recognition-Module-TCS230-TCS3200-/190558321419?pt=LH_DefaultDomain_0&hash=item2c5e293b0b to detect colours but it may not work in passive mode. A major problem would be reading the colour without interfering with the viewing of the picture. The above sensor would only detect colour in a small area and not the entire screen a camera could see the entire screen but is probably beyond the power of an Arduino to read and average the colour with any speed.