The distribution of UV-continuum slopes for the u-dropouts as measured by the r − i colour from the CFHT data (black) compared with the outcomes from simulations. The colour is measured in a part of the spectrum without strong features, redward of 1600 Å. Both an older template and a higher amount of dust result in a redder UV colour. The amount of dust we need to add therefore depends on the age of the base template. For the u-dropouts we find that a uniform distribution of dust with 0.1 < E(B − V) < 0.4 gives a good fit to the data, when the template is 100 Myr old with a constant amount of star formation (purple).