There's no truth in the old adage that white horn is weaker than dark at all. If it were true, then horses with one white foot would haev one foot that wore away faster than the rest - or horses with stripey feet would have really odd patterns of wear! It just depends on what part of the coronet is pigmented, as Mehitabel explained.
Stripey feet are a feature of some colours or breeds (Appaloosas, for example) and white feet can be a boon because bruising shows up on white horn and not dark - makes spotting potential injuries a lot easier! That has probably also contributed to the idea that white feet are weaker - it's not so at all, it's just that the pigment in dark hooves hides bruising.