Control a 3.6V motor (forward/idle/reverse) using two phototransistors facing a smartphone screen
I think that your project can be done very easily. 1)because you don't have an electronic background,  I will tell you to ignore the comments about noise being injected into the battery.  2) you are right about the screwdriver motor. It does give you more torque...considering that we don't know anything about your load, you might need it. 3) Those screwdrivers usually have a rechargeable battery and wall charger. If you have the one that came with the screwdriver,  really ignore the noise comment. 4) Do you know what a relay is? From digikey.com you buy a relay for your project. I will suggest that you buy a relay that can handle at least 5volts at 5A max. 5) your motor will be  connected to your relay. You will activate the relay with the output of a color sensor. There many color sensors out there. I do believe that there is a color sensor that gives you a 5v output when a black black line is detected. Check parallax.com or sparkfun or robotshop.com. 6) To recap, when the sensor sees  black on the screen, the output will go high, hence activating your relay, which will turn on your motor.  Good luck !