  We recently reviewed the experimental determination of the volume fraction,
$\phi$, of hard-sphere colloids, and concluded that the absolute value of
$\phi$ was unlikely to be known to better than $\pm 3$-6%. Here, in a second
part to that review, we survey effects due to softness in the interparticle
potential, which necessitates the use of an {\em effective} volume fraction. We
review current experimental systems, and conclude that the one that most
closely approximates hard spheres remains polymethylmethacrylate spheres
sterically stabilised by polyhydroxystearic acid `hairs'. For these particles
their effective hard sphere diameter is around 1-10% larger than the core
diameter, depending on the particle size. We argue that for larger colloids
suitable for confocal microscopy, the effect of electrostatic charge cannot be
neglected, so that mapping to hard spheres must be treated with caution.
