Tengo una idea genial.

Translation: I’ve come up with a brilliant idea.

Obviously, this can also be una idea brillante.


It seems that the two are very close, with brillante referring to something that is shiny, sparkly, bright - which can be what you mean by a “brilliant idea”. But then there is genial, which is masterly - also how you might describe your brilliance. My primary language is English, and I have to say that I have learned a lot about my own language during my Spanish learning experience. Taking a closer look at the meaning of words is very interesting. Thanks for the poke!