Differential phase is the error in the phase of the colour signal due to a change in luminance (brightness) level. It is measured by comparing two chrominance signals of initially equal phases riding on two different luminance levels. The inaccuracy is measured in degrees. The phase of the colour signal is responsible for displaying the proper hue, or base colour. Differential phase is important to video signals because if the phase of the subcarrier signal changes with a change in luminance level, then the picture being displayed will have colour selection problems.