The red dye is the 4,4'-bis(p-dimethylamino-phenylazo)-2-methyl-azobenzene; the yellow dye is the 4,4'-bis(p-methoxy-phenylazo)-2-methyl-azobenzene; and the blue dye in an anthraquinonic dye of formula (III), ##STR1## in which A is ##STR2## and B is H or ##STR3## where R and R', indentical or different, are a lower alkyl or alkoxy group, one of the A and B being H when the other is not H.