Table 2. National Bureau of Standards (NBS) system of expressing color differences [19,20].

∆E	NBS Criteria
0–0.5	Trace: Remarkably slight alteration
0.5–1.5	Slight: slight alteration
1.5–3	Noticeable: Observable alteration
3–6	Appreciable: Apparent alteration
6–12	Much: Remarkably apparent alteration
12 or more	Very much: Alteration to other color

NBS unit = ΔE × 0.92