Top panel: g - r (brown circles) and N_HI (green crosses) scaled (mean subtracted, sigma divided) values in each of the 33 apertures. A grey dashed line has been drawn at g − r = 0 for reference. Redder colours and higher N_HI are positive in this figure. Bottom panel: g - r median colour index vs. N_HI measured in the 33 apertures traced over the pseudo-ring. The green horizontal stripe represents the Green Valley in SDSS g - r.