Extinction law with linear (left) and logarithmic (right) x-axis. Top: green dots show the measured slopes of the RC stars using the F090W – F200W JWST colour. The solid line shows a univariate spline interpolation. Middle: red dots show the extinction curve as a function of F555W – F775W colours with results from De Marchi et al. (2016) (black dots) for optical wavelengths. Bottom: interpolated results for A_λ/E(V − I), compared to the MW extinction law (Fitzpatrick 1999) with R_V = 3.1, corresponding to R_V − I(V) = 2.3. Ratio and difference between those curves are shown by the orange dotted and purple dash-dotted lines, respectively.