Top: comparison of g (right panel) and r-band (left panel) magnitudes derived from the integrated spectrum within an aperture of 30″/diameter extracted from the V600 datacubes and the corresponding magnitudes derived from the SDSS images, before (blue solid circles) and after (red solid squares) the flux calibration described in the text. Bottom: comparison of g − r colour. Considering the errors in the absolute flux calibration (~8%) and between g and r band (around 4–5%), the expected errors in the colours are  ~0.1 mag, consistent with the errors in SDSS photometric data.