Rainbows are a mix of a couple different genes. To be a true rainbow your birds needs to be opaline,blue series,clearwing,yellow face type two.
Opaline is a sex linked gene. To produce an opaline you will need a opaline male or a male that is carrying opaline. Blue series is recessive to green. So it would be easer if you had two blues. you could have a green but you would need it to carry blue to produce blue series. Clear wing is recessive to most other wing patterns. So you would need two clear wings or carrying the gene. Yellow face type two is complicated but dominant over white face. You would need one parent that is yellow face type two.