“The colour of a placebo can influence its effects. When administered without information about whether they are stimulants or depressives, blue placebo pills produce depressant effects, whereas red placebos induce stimulant effects (Blackwell et al., 1972). Patients report falling asleep significantly more quickly and sleeping longer after taking a blue capsule than after taking an orange capsule (Luchelli et al., 1978). Red placebos are more effective pain relievers than white, blue or green placebos (Huskisson, 1974; Nagao et al., 1968).”
From Prof Irving Kirch’s chapter on placebo in The Power of Belief: Psychosocial Influences on Illness, Disability and Medicine (ISBN 0198530110).