As with humans, genetic features are random and there is no guarantee of what the offspring will look like.
You weren’t specific with your question but I am guessing you want to know where the coloring of the sandy pup came from, and there are two possibilities.
The first is that the female snuck off and mated with a sandy colored male. Dogs can conceive throughout their heat cycles so if multiple males ejaculated inside the female, then it is possible that her eggs were fertilized by the sperm of multiple males; hence, it is possible for her to crank out pups who have different sires in the same litter.
The second is that one of the dogs has the sandy-colored gene as a recessive trait and it cropped up in that one pup.
The sandy-colored ancestor could be several generations back so there is no telling where it came from.
This is similar to a couple with brown hair who pop out a red-haired child. Red hair is a recessive trait and may ‘skip’ generations so the parents look at their kid and are stumped because no one in either family has red hair.
What it really means is that no one in their family who is currently alive has red hair but somewhere back on the family tree of one of them there is a redhead who, once into the family, contributed this genetic trait.