“Deep periwinkle, plumbago, or the electric blue of the gloaming hour from his native England”: Those are the words of a professor with the University of Massachusetts, Amherst, Peter Gizzi, trying to pin down the exact variation of blue in one of Trevor Winkfield’s paintings. What is it about Mr. Winkfield’s art, currently the subject of an exhibition at Tibor de Nagy Gallery on Manhattan’s Lower East Side, that makes his admirers wax poetic?