A rainbow is a composite bird and one of the hardest mutations to consciously produce. Breeders in ABS or BAA may be able to give you a few tricks. See their web sit and ask for help from any breeder with rainbow experience. I have had many '3's' But getting that 4th in a competitive show bird is hard.
Dan Schultes in Iowa breeds them. See contact info for breeders/board members on BAA site. I currently have a yellow face, opaline, sky white dilute that could fool a novice but it does not have clear wings. nor is it a grey wing
It was a tantalizing gift as a chick as the line that produced it had it hidden.
Sunny Sky is a favorite of visitors and has fooled more than a few wishing for a rainbow.