The reflector idea won't work well as colour sensors are pretty directional by themselves. The reflector would either need an omnidirectional sensor, or a second inverted reflector that goes in place of the light bulb (the focal point of the reflector), reflecting the light as a parallel beam on to where the base of the bulb was. That then can be picked up by a colour sensor. A short narrow tube around the sensor, and the sensor moved back and away from the reflector a bit should take care of most ambient light, at least to the extent that it's not a significant influence any more.