Vectorscope

Colour is plotted as a position around the circle and a distance from the centre. Direction gives the hue, distance gives saturation, and a completely grey image sits as a dot in the middle. Colourists use it to spot casts a monitor is hiding and to match shots that should belong to the same scene. The skin tone indicator, a line running towards the upper left, is the one most creators reach for, because human skin of every tone lands close to it and a face drifting well off that line is usually a white balance problem.