Interesting thought Grif - Herps do have pretty amazing eyes; and some species of reptile have superficial pineal glands that allow them to 'see' UVB/C spectra in addition to UVA rays.

I do believe most frogs can distinguish color fairly well; and just as you say it can vary species to species. I recall a short film (the name eludes me) that was introduced to me during a lecture on perception and conditioning to an extent, where a toad refused to eat a hornet after being stung, and still refusing to eat anything yellow and black with warning coloration. Nocturnal species for example must have also exceptional vision in the night but UVA may not be as readily available... in this case a white insect against a dark background to create a contrast is going to be a prime target I would imagine. I'm really only hypothesizing myself though = )

I've also had reptiles show preferences to colored meals - or have no problem feeding on soldier grubs or silkworms who are also white in coloration. My bearded dragon prefers things that are red in color, making the E. posticus roach nymphs and certain red fruits such as raspberry particularly appealing to him, lol.