Table 2 The color changes in L*, a* and b* axes (expressed as ΔL, Δa, and Δb values) at each experimental time point (HAP 1–3, HSF)

ΔL

10 wt% nano-HAP

2.30 (1.12)^aA

2.70 (1.12)^aA

2.76 (1.36)^aA

1 wt% nano-HAP

1.13 (0.73)^bB

1.51 (1.20)^bAB

1.78 (1.70)^aA

Without HAP

− 0.01 (0.42)^bA

− 0.13 (0.68)^cA

− 0.08 (0.73)^bA

Water

0.01 (0.01)

0.01 (0.02)

0.00 (0.04)

Δa

10 wt% nano-HAP

− 0.21 (0.21)^aA

− 0.75 (0.90)^bB

− 0.95 (0.86)^bB

1 wt% nano-HAP

0.05 (0.47)^aA

− 0.20 (0.72)^abA

− 0.31 (0.83)^abA

Without HAP

0.05 (0.33)^aA

0.09 (0.40)^aA

0.12 (0.36)^aA

Water

0.01 (0.02)

− 0.04 (0.04)

− 0.03 (0.06)

Δb

10 wt% nano-HAP

− 1.34 (0.64)^bA

− 2.55 (1.84)^bB

− 2.66 (1.90)^cB

1 wt% nano-HAP

− 0.84 (0.63)^abA

− 0.96 (0.83)^aA

− 0.10 (0.83)^bA

Without HAP

0.03 (0.47)^aA

0.21 (0.96)^aA

0.25 (0.96)^aA

Water

0.00 (0.03)

− 0.01 (0.04)

0.02 (0.04)