Why might "proud" be correct in a slogan and not "proudly"?
In the movie Great Dane Marmaduke, someone says, "Say it loud. Say it proud."
But proud is an adjective, whereas the sentence really needs an adverb, proudly. Perhaps the same applies to loud as well.
Why is "Say it proud" right?
Proud is an adjective, proudly is an adverb.
"I say it proud" means I say it, and I am proud while I am saying it.
"I say it proudly" means I say it in a proud way.
"Proud" means it is me who is proud. "Proudly" means it is my speech that is proud.