  For many years broadband colours have been used to obtain insight into the
contents of galaxies, in particular to estimate stellar and dust content.
Broadband colours are easy to obtain for large samples of objects, making them
ideal for statistical studies. In this paper I use the radial distribution of
the colours in galaxies, which gives more insight into the local processes
driving the global colour differences than integrated colours. Almost all
galaxies in my sample of 86 face-on galaxies become systematically bluer with
increasing radius. The radial photometry is compared to new dust extinction
models and stellar population synthesis models. This comparison shows that the
colour gradients in face-on galaxies are best explained by age and metallicity
gradients in the stellar populations and that dust reddening plays a minor
role. The colour gradients imply $M/L$ gradients, making the `missing light'
problem as derived from rotation curve fitting even worse.
