The lightness of four colored samplesmred, green, blue, and yellowmwere compared with a gray scale while viewed under different levels of illumination from 0.01 to 1000 lux. The use of different field sizes allowed a differentiation between the effect of the rods and cones and thus between scotopic and photopic vision. For scotopic vision, rods only, the equivalent lightness on the gray scale increased with the level of illumination, reflecting the increase in the effect of colorfulness on brightness. For photopic vision, rods and cones, a similar effect is observed but is attributed to a combination of changes in achromatic and chromatic response. The results demonstrate that, under conditions where both rods and cones are participating in vision, the equivalent lightness for blue and green samples decreases with increased illuminance for illumination levels less than approximately 3 lux and increases with light level at higher illuminances. This change in behavior at around 3 lux marks the transition between scotopic and photopic visual mechanisms.