Estimate of the age of the extraplanar HII regions using a colour index. Left panel: variations of the synthetic Hα-FUV age-sensitive colour index as a function of time derived for the star formation history given in Eq. (4). The three dust attenuations are shown as E(B − V) = 0.0 (blue), 0.1 (green), and 0.2 (red). The black solid line shows the median of the observed colour distribution of the star complexes in the tail of IC 3476, the grey lines the 16%, and the 84% quartiles of their distribution. Right panel: observed distribution of the Hα-FUV colour index.