Delta-E is a single number representing the "distance" between two colors.
It's tempting to simply compare the euclidean distance difference between the red, green and blue aspects of an rgb.
Unfortunately for us rgb was intended for convienient use with electronic systems, however it doesn't align with how we actually perceive color.
Testing this method quickly reveals sporadic and often drasticly different results than one would expect of visually "similar" colors.
Enter CIE76...