Maneuver to Center of Target w/ Phototransistor


I made a custom robot with two continuous servos for the wheels, and I'm trying to figure out the best way to maneuver it to the center of a multicolored target using a phototransistor. Right now, I have the phototransistor set up to detect the different colors, but I'm troubled about how to make the one phototransistor control the movement of the robot so it will reach the center (the target is a circle). The transistor is located at the front of the robot, and has it's own light source to ensure the phototransistor reads consistent values. The target has 5 colors before reaching the center, and the floor is a solid color not in the range of the target. I'm thinking I can swivel the robot back and forth to test the colors and make sure its driving toward the center, but I'm not sure if that will work... I'd be interested to hear any suggestions.

Unless you can move the sensor itself around on a servo or similar, I think you'll probably have to swing the robot around to help determine which way to go.

You say "it's own light source".
Are you doing this with a single light source?
Or are you using coloured LEDs, activated one after the other, to allow the phototransistor to detect individual colours by reflection intensity?