Throughout the experiment, colour readings were evaluated at five positions on each head daily. L⁄
was lightness (0–100: 0 = black, 100 = white). The H value (hue angle) was calculated as H = tan1 (b/a) when a > 0 and b > 0 or h = 180 + tan1 (b/a) when a < 0 and b > 0.