wvmarle:
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.
Plus you would want to remove any interferences like clouds, trees or parts of whatever is holding the sensor. a 15 degree circle is popular among various camera's light meters (which have accounted for color since the late 70s.) Sky color is usually measured in kelvins, reducing less actinic stray light to single digit percentages.