  (Abbreviated) Optical observations of a statistically complete sample of
edge-on disc galaxies are used to study the intrinsic vertical colour gradients
in the galactic discs, to constrain the effects of population gradients,
residual dust extinction and gradients in the galaxies' metal abundance. It
appears that the intrinsic vertical colour gradients are either non-existent,
or small and relatively constant as a function of position along the galaxies'
major axes. Our results are consistent with the absence of any vertical colour
gradient in the discs of the early-type sample galaxies. In most galaxies
small-scale variations in the magnitude and even the direction of the vertical
gradient are observed: at larger galactocentric distances they generally
display redder colours with increasing z height, whereas the opposite is often
observed in and near the galactic centres. For a significant fraction of our
sample galaxies another mechanism in addition to the effects of stellar
population gradients is required to explain the magnitude of the observed
gradients. The non-zero colour gradients in a significant fraction of our
sample galaxies are likely (at least) partially due to residual dust extinction
at these z heights, as is also evidenced from the sometimes significant
differences between the vertical colour gradients measured on either side of
the galactic planes. We suggest that initial vertical metallicity gradients, if
any, have likely not been accentuated by accretion or merging events over the
lifetimes of our sample galaxies. On the other hand, they may have weakened any
existing vertical metallicity gradients, although they also may have left the
existing correlations unchanged.
