Single post in Gray/grey out pokemon in the undiscovered group in the daycare

If you want to grey out legendaries in the daycare like the title of your suggestion says, then they'd have to own the legendaries before they could see that they're greyed out and therefore unbreedable - meaning they already own the pokemon, and rendering your point moot. This is also all assuming that the newbies don't know the first thing about legendaries and can't read the wiki or use the help forum.