The present invention relates to an unsaturated polyester resin or vinyl ester resin composition, wherein the unsaturated polyester resin or vinyl ester resin composition comprises an unsaturated polyester resin or vinyl ester resin and a copper compound and an acetoacetamide compound having the following formula: whereby R1, R2, R3, R4 and R5 each individually may represent hydrogen (H), or a C1-C20 alkyl, C6-C20 aryl, alkylaryl or arylalkyl group, that each optionally may contain one or more hetero-atoms (e.g. oxygen, phosphor, nitrogen or sulphur atoms) and/or substituents; a ring may be present between R1 and R2, R2 and R3, R3 and R5 and/or between R4 and R5; R4 and R5 may be part of a polymer chain or may be attached to a polymer chain; and the resin being curable with a peroxide and the resin composition contains less than 0.01 mmol cobalt per kg primary resin system and less than 0.01 mmol titanium per kg primary resin system.