The total color difference computed with a color difference equation. It is generally calculated as the square root of the sum of the squares of the chromacity difference, Delta C*, and the lightness difference, Delta L*.

Generically, the compound degree to which two colors appear to match. Usually calculated in CIELAB, but other color-differencing systems exist. See example below.

In 1925, Reichsauschuss fur Lieferbedingungen (RAL) was founded in order to establish industry standards for the German paint industy, the the RAL System published in 1927. Then in 1931, the CIE system was published - a system that defined colors by three values: L for light (black to white), a* for red to green and b* for blue to yellow. See CIELAB

With these metric systems in place, a color difference could e defined a "Delta E" calculated as the square root of the sum of the squares of the chromacity difference a+b and the lightness difference L. This system has continued to improve up to the present day, but the standard principles were established 70 years ago. These historic developments all lead to the establishment of color management standards.

Color Management involves determining the tristimulus values of X, Y, and Z, while allowing for the characteristics of human color vision. From this we can calculate tristimulus coefficients, classify a color according to the various color systems, match it with a similar color sample and, finally specify matching pigments mixtures.

For example: when we refer to the color being a certain deltaE value, we typically recognize that a deltaE<2 is a match were the trained eye may not even notice the difference; a deltaE between 2-5 is a close match were most observers would say there is a slight difference but it is still an acceptable reproduction; a deltaE between 5-10 most observers would say the color is not a match, but the reproducibility level is up to the observer; and a deltaE >10 is noticeably not a match. In the case where the deltaE reports the color as being a non-match, modifying input profiles and output profiles for media may give better results.

To help eliminate this problem, color scientists have over the years developed new formulas to determine Delta-E that more accurately reflect how the eye sees color. The formulas available to us are Delta-E CMC, Delta-E 94, and Delta-E 2000. Each of these formulas do a better job of making sure that a Delta-E difference with a specific value will look the same no matter what colors we are comparing.

If you take two colors, measure them with your densitometer and plug the L*a*b* values into the following formula:
E =  (L1 - L2)2 + (a1 - a2)2 + (b1 - b2)2
The resulting number is referred to as Delta E. See more information about Delta E 2000 below.

Delta E (CIE 2000)

The color difference, or ΔE, between a sample color L₂a₂b₂ and a reference color L₁a₁b₁ is:

http://www.brucelindbloom.com/index.html?Eqn_DeltaE_CIE2000.html

Implementation Notes:

1. The angles supplied to the sin and cos functions are shown in degrees. Most math libraries expect radians, so don't forget to convert.
2. The inverse tangent is also expressed in degrees. In most math libraries, the inverse tangent returns radians, so don't forget to convert.
3. In computing hue angles, be careful with the inverse tangent since a could be zero. Instead, use special math functions to do this. In both the Standard C library and Java, this function is called atan2. In Microsoft Excel, it is called ATAN2. These special functions will compute the proper inverse tangents without needing to worry about "divide by zero" conditions.

Delta (D) - A symbol used to indicate deviation, difference or change.