Flux ratios of the blue and red filters when the Hα emission can be measured in a complex. The broad-band filter fluxes have been multiplied by the band-width to obtain the values with the same units as the Hα flux. Solid line shows the evolutionary track of the same ratios for a single burst using SB99 models (see text). Dotted line shows the track for a composite population where 99% of the mass corresponds to a 1 Gyr population. The small triangle almost at the beginning (zero-age young population) of the second track indicates that the young population is 2 Myr old and we add 1 Myr to the subsequent triangles. The typical uncertainty associated with the data is shown above the extinction vector, which is drawn at the bottom right-hand corner.