Context. The taxonomic classification of SDSS observations of asteroids show a dependency on the phase angle of the observations. Aims. We aim to quantify this dependence and use Hapke models to investigate its cause. Methods. Statistical methods were used to characterize the taxonomic variation with phase angle. The effect of several Hapke parameters on the spectral slope and band depth of synthetic spectra for a slab and for ellipsoids with different oblatenesses were then analyzed. Results. Effects linked to body shape and the solar phase function and macroscopic roughness can alter key spectral parameters that are related to the taxonomic classification depending on the solar phase angle, rotational phase of the observation, and shape of the asteroid. Depending on the oblateness of the body, shape effects are similar to phase effects.