The instrument was standardised each time with a black and a white (L = 91.10, a = 1.12, b = 1.26) tile. The colour values were expressed as L⁄ (lightness), a⁄ (redness/greenness) and b⁄ (yellowness/blueness). Total colour difference (TCD⁄) was calculated and modelled. Colour values were the means of triplicate measurements.