Fig. 5. (A) Achromatic contrast between four stimuli (the white evening primrose, the red hindwing of D. elpenor, the yellow flower, and the blue flower) and a green leaf background. Positive contrast indicates that the object is brighter than the background. (B) Chromatic contrast, defined as the distance between the relative quantum catches of the stimuli and the leaf background. (C) Chromatic contrast assuming that D. elpenor has von Kries color constancy and is adapted to a background of green leaves under each illuminant. (D) Coefficients of variation of the achromatic and chromatic contrasts of the four stimuli when viewed under nautical twilight, moonlight and starlight.