Examples of suitable compounds B) of the type aa) are ketones, such as acetylacetone, benzoylacetone or acetyldibenzoylmethane, and also esters of an optionally alkyl-substituted acetoacetic acid, such as ??-methylacetoacetic and/or ??-methylacetoacetic acid, or of acetonedicarboxylic acid, malonic acid units, attached by an ester linkage, of malonic acid and monoalkyl derivatives thereof, linear