The disappearence of white dog poo is due to what dogs have in their diets these days.
The white doggy doo is mainly due to water evaporation leaving calcium behind. The inorganic components are left behind due to the organic bits being consumed in various ways.
Today's modern dogs don't eat anywhere near as much bone as they used to, including bone meal. Butchers just don't give them out any more. The BSE crisis was to blame for that. Today's much tighter regulations on dogs doing their business on our pavements means that droppings don't hang around for years in public places like they used to, giving them less opportunity to dry out and turn white
.... and that concludes the Fido memorial lecture for this year from the Turdmeister