Spotted describes something covered with small round marks of a different colour — a spotted dog, a spotted shirt, a spotted egg. The spots are part of how the thing looks, its pattern, not dirt. You use it in front of a noun very comfortably: 'a spotted tie'. For clothes and fabric, British English says spotted where Americans usually say polka-dot, and both languages also reach for dotted. Be careful with animals: leopards are described as spotted, but a cow or a horse with big irregular patches is usually patchy or, for horses, piebald. There is a second life to this word that trips learners up — it is also the past form of the verb spot, meaning to see or notice something, especially something hard to see: 'He was spotted leaving the building.' Same spelling, completely different job, and context is what tells you which one you are looking at.