Recent guides for use of sunglare protection filters have introduced the concept of Q factors as a measure of colour appearance distortion in viewing traffic signal lights. The adoption of Q factor values was apparently arbitrary and not firmly based on experimental data. The manner in which changes in Q factor affect colour perception and detection of signal lights has been measured and shows that detection thresholds vary with the Q factor in a manner that can only partly be explained, but that is not independent of the colour of the signal as assumed in the guides.