The Din99 Delta E is an improved color difference metric, but
rather than being based directly on L*a*b*
delta E, it uses a modified L*a*b* space called DIN99 Lab, and is a
simple Euclidean difference in that space.
Since it is based on L*a*b* space, a reference white needs to be
chosen to compute this Readout value.
Details of white point configuration.