The best sensor for detecting a coloured ball would be an NXTCam
from Mindsensors, a US based company, however there very expensive and not easy to program.
I am not sure a light sensor will be any good at finding a ball but once you have it you will be able to determine its colour if necessary.
All of that is probably not very useful. I would be tempted in your situation to invest in a roboball
for Robocup football. Robocup is an international robotics competition so it might open more opportunities. You can purchase these form HiTechnic as well as a specific sensor to detect the ball.