Can you use 'Monster Reborn' on 'Rainbow Dark Dragon'?

"Monster Reborn" cannot be used to Special Summon "Rainbow Dark Dragon" from the Graveyard even if the monster was Special Summoned following the conditions on the card before being sent to the Graveyard.

That's because it has the text "cannot be special summoned except by", meaning the listed text is the only way it can ever be summoned.

This is not to be confused with monsters like Chaos Sorcerer, who says "can only be special summoned by...". These monsters are the ones that can be revived from the graveyard, but only if properly summoned first by their listed method.