Desaturating the image can help with determining a light source. When looking at an image in only grayscale values, inconsistencies with plotting cast shadows can indicate incongruent light sources. If people are outside, they are all illuminated by the sun. The person whose shadows, don't follow the sun is the one who is out of place in the photo. Low sat Bernie is lacking chroma because of atmospheric perspective outdoors and being shot from a distance. Indoor photos would be full saturation as they are closer to the film plane.