The contrast between the marks and the background color of the model mandarins was measured using color photographs of marked models. Photographs were taken of marked models using a D5100 single-lens reflex camera (Nikon, Tokyo, Japan) mounted on a Kaiser RS2 copy stand (Kaiser Fototechnik GmbH & Co.KG, Buchen, Germany). Models were placed on a black velvet background and oriented such that the mark was facing the camera. Illumination was provided by two D-Lite RX One flash units (Elinchrom SA, Renens, Switzerland) fitted with Portalite softbox diffusers. The camera was operated using ControlMyNikon v. 4.3 (Nikon, Tokyo, Japan), with images saved in Joint Photographic Experts Group format, with dimensions of 3264 × 4928 pixels, and with three color channels (red, green, and blue, RGB). Images were captured in a single session.
The color of marks was measured from each image using FIJI (
Schindelin et al., 2012) (
http://fiji.sc). The mean values for each RGB channel were recorded from a manually defined region of interest (ROI) encompassing the mark. In addition, ten ROIs (100 × 100 pixels) were randomly placed throughout the area of the photograph covered by the model. RGB values were converted into the L* a* b* color space (
Carter et al., 2018) using the ‘colorspace’ library in R (
Zeileis et al., 2019). The luminance (L*) parameter of each ROI was used to calculate the Weber contrast (
\(C = \frac{{{{L}_b} - \ {{L}_m}\ }}{{{{L}_b}}}\)) between the mark (
Lm) and the background (
Lb).