Color variation has been reported within many species of Lepidoptera, and may be important in larval thermoregulation. Researchers have reported seasonal melanization and lack of yellow pigmentation in monarch larvae (Danaus plexippus L.). While the latter results from a recessive autosomal mutation, the source of variation in larval melanization is unclear. We measured the proportion of black, white, and yellow coloration on monarch fifth instars when larvae were reared in warm (32–22°C), control (27–17°C), or cold (20–10°C) temperature treatments. We found that temperature had a significant effect on monarch larval coloration. Larvae reared in a cold environment had more black and less white and yellow pigment than those reared in a warm environment. Because coloration affects absorption of radiant energy, color variation in response to temperature may be an ectothermic adaptation to suboptimal thermal conditions.