Like humans, hawkmoths have three different types of photoreceptors used for photon acquisition (gathering of light) within the eye.  But unlike humans which have red, blue, and green photoreceptors, hawkmoths have a photoreceptor in the ultraviolet range instead of red.  Based on a series of optical, anatomical, and physiological data and complicated equations, the general effectiveness and contribution of these three photoreceptors (blue, green, and ultraviolet) were put to the test.
 
Based on the results, it was determined that the effectiveness and contribution of the ultraviolet photoreceptors was quite low in relation to the other two (blue and green photoreceptors).  So an additional test was carried out by the researchers to see if these ultraviolet photoreceptors contributed to the hawkmoth’s color vision at all. 
 
Results:

Three moths were trained to discriminate a white stimulus that reflected ultraviolet light from one that absorbed it.  The reward color was selected to be the ultraviolet-absorbing white stimulus instead of the ultraviolet-reflective.  The moths chose this reward color (ultra-violet absorbing) exclusively for 23 out of 23 tests conducted.
 
Conclusion:

What this tells us is that the ultraviolet photoreceptors do play a role in the gathering of light and color determination within the eye of the nocturnal hawkmoth.  Another point of consideration suggested by these results is that hawkmoths may posses trichromatic color vision, just like humans, only further tests will be needed to establish this.