Abstract. In the sexually dichromatic sand lizard the area of the nuptial green coloration in males is allometrically related to body mass, and males with higher fighting ability have more saturated nuptial colours. Experiments in the laboratory showed that the extent of the bright nuptial coloration in males affects their contest behaviour; manipulated males with more colour were more likely both to initiate contests and to win. Further, when manipulated males had badges of unequal size the contests were settled more quickly than when the badges were the same size. This predicts that superior fighters with larger badges should have higher mating success; this was confirmed by field studies of a natural population. © 1994 The Association for the Study of Animal Behaviour.