The results from the measurement of ΔE value at different pH.

Specimen	Standardized indicator for the perceived color difference by an observer (ΔE)
	pH = 2–4	pH = 4–6	pH = 6–8	pH = 8–10	pH = 10–12
S1	6.08	13.32	15.95	25.09	42.7
S2	2.78	8.07	22.22	32.23	40.8
S3	1.36	9.54	26.53	38.21	44.3
S4	3.42	7.27	9.47	11.09	13.5