Color difference has a lot of different formulas that quantify the color difference in various ways.

∆E

The most common is ∆E which can be expressed with numbers calculated using various formulas:

• ∆E 1976
• ∆E 1994
• ∆E 1994 for textiles
• ∆E CMC (1:1)
• ∆E CMC (2:1)
• ∆E 2000 (currently recommended for the print industry)

None of those single-number formulas is perfect - in most cases, the observer's impression doesn't correlate perfectly with the numbers - in some situations covered color range seems to be different depending on the direction of color variations. 

Other common deltas:

• ∆C - saturation difference
• ∆h - hue angle difference - don't apply on neutral or near neutral colors
• ∆H - hue difference (linear distance in a*b* plane)
• ∆L - lightness difference
• ∆a - linear difference on the magenta-green axis
• ∆b - linear difference on the yellow-blue axis
• ∆C_h - 

Multi-axes (Snowflake) Tolerances

This alternate solution is based on tolerances defined for six directions (3-axes). As no one perfect ∆E formula can be easily adopted solution is based on more - than one number, which brings more flexibility in some cases.

Other important metrics 

Some phenomena ( Metamerism or Fluorescence) can be hard to control when lighting conditions are different than standardized. Invisible UV components can dramatically change how the sample is perceived. Due to different pigmentations/colorants/dyes, two samples might match in one Illuminant and be dramatically different under another.

The following variables can help solve those potential problems:

• OBA Index (∆b_M1-M2) — reflects the amount of Optical Brightening Agents
  Note: if this index is higher than 1-2, an M1 M-condition should be used!
• Fluorescent Index —
• Metamerism Index —