Top: (g − r) colour of the background QSOs as a function of redshift. The black points represent the DR11 QSO sample. QSOs with foreground ESDLA that have a Ly α centroid that falls in the u, g, and r-band are represented by purple, green and red points respectively. The blue line shows the change in the median colour of the overall QSO sample as a function of redshift. Bottom: normalised distributions of colour excess Δ(g − r) = (g − r) − ⟨g − r⟩_z for the DR11 QSO sample (black hashed histogram) and the QSO with foreground ESDLAs (red unfilled histogram). The vertical lines mark the medians of the two distributions.