Fig. 4. The use of chromatic vision. (A) The reflectances of a typical yellow and a typical blue flower and a green leaf. (B) The spectral composition of direct sunlight and skylight in a shadow, measured shortly before sunset, on a summer day in Utah, USA. (C) The achromatic contrast between both flowers and the green leaf in both illuminations differ dramatically for the green receptor of D. elpenor. (D) The colour loci of all three stimuli in the colour triangle of D. elpenor are rather constant in both illuminations, even without the assumption of colour constancy. UV, UV light; B, blue light; G, green light. (For details and methods of all calculations, see Johnsen et al., 2006.)