Is this Legal for tournament?

I want to rebuild my deconstructed [Sauron, The Dark Lord] deck with the reintroduction of [Gifts Ungiven]. The majority of that deck is dispersed into three other decks… Can I label the outside of a perfect fit sleeve and use the same outer color sleeve? With that idea, does that red flag legality in tournament because the card now becomes “labeled”? I’m sure many people have brought this up I just want to have 4 high power decks but I don’t want to spend hundreds of dollars on multiple copies of expensive cards at the moment.