I am not expert but here is a thread from a forum: "(Fly and Modena) are not the same colours. Giallo Fly is a deeper, richer Yellow than Giallo Modena, although everyone has to admit that its very hard to tell if cars in both colours do not stand close enough to identify the colour."
My experience is that Modena is an "older" paint name and that Fly was struck in the 60s as something quite catchy and was initiated with the 275 GTB. The shade specifics are beyond me 