Some crab spiders can change their colour to match the flower they use as a hunting platform and, in choice trials, they select same-colour flowers over contrasting flowers. Colour change is a costly physiological process that could help spiders capture more prey or avoid predators. There is no evidence, however, that crypsis increases the hunting success of spiders, and its effect on predator avoidance has not been studied. To evaluate the effect of crypsis on predation rate we tethered yellow crab spiders, Thomisus onustus, to artificial white and yellow flowers. To evaluate its effect on hunting success we released them on yellow corn daisy, Glebionis segetum, inflorescences and purple common mallow, Malva sylvestris, flowers. The colour contrast between crab spiders and artificial flowers was well above the detection threshold, but it was twice as large for spiders tethered to white flowers. Overall predation risk was higher on white than on yellow flowers. Yellow spiders released on purple mallows were three times more likely to capture prey than those released on yellow daisies. Despite this difference, during field surveys we have never seen a Thomisus female hunting on a mallow flower. Colour match plays a role in predator avoidance. Imperfect crypsis, that is, not matching a flower exactly, still more than halved predation rates on crab spiders and, in the field, spiders selected flowers with colours they could match, ignoring others where their hunting success would be much higher, but where they would be extremely conspicuous.