When we say 'rainbow', to what degree of 'rainbow' are we referring to?
Are we talking about compositions where the eidolon you want for the passive is a dual-element (Apocalypse/Phoenix/Fafnir/etc.)?
Or we're talking about a team where one of the standard elemental lilim looks good?
Or a team where the illusion/phantom lilim is the choice? (when you have/want no more than 2 kamihime of any given element)