It should be Topsy. Ducky's parents are lavander too. Littlefoot's grandparents are a lot more purple as well. It's like the animators used a different colour pallete to draw the Great Valley and the adults, and another when adding the kids latter on.
Why would they do this is beyond me. Well, not quite. Animating is hard work, and if the studio lacked perfect organisation or a good work atmosphere errors like this are bound to happen.