Upper panels: colour magnitude diagrams of y₀ vs. δ₄, c_y,0, (b − y)₀, c_1,0, m₀ and (u − y)₀, respectively, where the colours were detrended by subtracting a ridge line polynomial (for δ₄ this is shown in Fig. 8). Symbols as in Fig. 8. The purple lines demonstrate the differential effect of He enhancement by ΔY = 0.15. They correspond to 13 Gyr, [Fe/H] = −2 dex, [α/Fe] = +0.4 dex Dartmouth isochrones, with Y = 0.25 (solid purple line) and Y = 0.40 (dashed purple line), respectively, from which the average colour at a given magnitude has been subtracted. The cyan vertical lines around y₀ = 18.3 mag show the expected difference between C-normal, N-poor (dashed line) and C-normal, N-rich stars (dotted line) with M_V = −1 mag taken from Carretta et al. (2011). Lower panels: histograms of the colour distribution of all stars in the clean RGB sample (light histograms) and of stars with 17.2 <y₀< 19 mag (dark histograms). The intrinsic spreads σ_intr obtained by assuming a Gaussian intrinsic distribution are reported and illustrated by the red errorbars above the histograms.