Is the part that still carries the color of animal droppings (buffalo, cows, donkeys) after being washed and dried considered impure?

The droppings of animals whose meat is permissible to eat (such as buffaloes and cows) are pure. As for the droppings of animals whose meat is impermissible to eat (such as donkeys), they are impure and must be washed until their traces of color or smell are removed, unless it is difficult to remove the color, in which case it is excused.