CMD K_s vs J − K_s for the b118 tile, illustrating the slope determination for RC1 and RC2. The black parallelogram outlines the selection box for the RC stars, with divisions indicating the colour cuts used for the GMM analysis. Yellow and white points represent the mean values of the Gaussian models for RC1 and RC2, respectively. The red and blue lines show the fitted linear trends for each feature, from which the slopes are derived. The colour bar represents stellar density in the diagram.