The correct answer is: Mr. Sargent bought a sweater for his dog with purple and pink spots.
The fact that the phrase "with purple and pink spots" comes after the word dog creates and ambiguity issue, implying the <em>dog</em> actually has colored sports embedded into its skin.
The adjective phrase should be written after the word <em>sweater</em>, since a sweater is more likely to fit that description than a dog.