NEW YORK — The classical image of a medieval court jester is a lithe figure wearing a bi-colored hat known as a cap ‘n’ bells and a colorful top over form-fitting breeches. While the Oxford English Dictionary defines motley as a medley, the term is also the name of the jester’s traditional costume made from bright and mismatched patterns and colors. April Fool’s Day provides an excellent opportunity to explore a popular collecting niche: Royal Doulton jesters.