Prisims separate light, such as that form the sun, by wavelength.

Prisims separate WHITE light, such as that form the sun, by wavelength.

Smell of the coloring agent used to color drab.

Explanation:

The  iguana prefers brightly colored insects over those with drab colors because though all the drab were from the same species but the one marked with red color might have special smell because of the coloring agent used.

Since it has already been mentioned in the question itself that iguana are more attracted to bright color insects as compared to dull color insects. So, the only possible reason could be some distinctive characteristics of the paint which is used for coloring the drab.