A process according to claim 1, wherein the quinophthalone dye is of the formula: ##STR11## wherein R' is a hydrogen atom or a hydroxyl group; each Y independently is a hydrogen atom, a halogen atom, a --CO--OR4 or a --CO--NHR4 group, where R4 is an alkyl group containing not more than about 10 carbon atoms; and X is a sulfur atom or a carbonyl group.