This is the dye A in which R3 is a butyl group and R1, R2 and R4 are each a hydrogen atom, this being the dye of Formula I in which Q1 is a 2-(1,1-dimethylethyl)-7-diethylamninobenz b!-4-pyran-4-ylidene grouping, Q2 is a 4- 2-(1,1-dimethylethyl)-7-diethylaminobenz b!-4H-pyrylium! grouping, R3 is a butyl group and R1, R2 and R4 are each a hydrogen atom.