What's the best way to use non native gems ?

[Arryn]

Daemon, why do you consider the expense of path boosters a "waste" of gems? As for rainbow pretenders, it all depends on what nation you are playing and what your particular play style/strategies are. If you are an aggressive expansionist, a rainbow is obviously not an optimal choice, especially if you have good national mages. OTOH, if you think you'll have the luxury of slow, methodical expansion and site searching, then a rainbow might well be a good choice (as having one will help your research and gem income quite a bit). You also haven't said whether you play SP or MP, and that makes a huge difference.

You seem to be looking for a one-size-fits-all answer, and this isn't the sort of game where that applies.