Boxplots of the J - H, H - K_S, and J - K_S colour distributions for the selected P4 sample. Each box represents the IQR (25th to 75th percentile), with the central red line indicating the median. The whiskers extend to the most extreme data points that are not considered outliers. The individual points beyond the whiskers represent outliers, i.e. values that deviate significantly from the main distribution.