The rainbow mutation is a mix of a couple different mutations and they all work differently. The mix is clearwing,blue series,yellow face type two, opaline.
Clearwing is a recessive mutation. This means that you either need both parents showing it or split for it.
Blue series is recessive to green series. This means that you need two blue birds or split for blue.
Yellow face type two is a dominant gene. You only need one parent that is yellow face type two. But i you have both parents with it the yellow will be brighter.
Opaline is a sex linked gene. You will need a male that either is opaline or is split for it to get female chicks to show opaline the male chicks will not show it just be split for it. But if you have both male and female opalines you will get male and female opalines.
it is not a true rainbow but a yellowface type two opaline spangle is easier to produce and look equally as beautiful. you only need a blue male that is split for opaline or opaline and one parent to be a yellowface type two spangle. both birds will have to be blue. only one parent has to have the spangle and yellowface gene but both being yellowface will make the yellow brighter.