All illusions are in your mind, so if you fell for it, you "see" what its fooling you to see.If you can see an illusion because light is reflecting off it, then the spell creates physical matter. The spell can't create physical matter according to the rules.
If you can see an illusion because it's producing light, then it creates light, which is against the rules.
All illusions are invisible.
Solves (for different variations of the term solves) most problems. IMO.