I just make my regular buttercream recipe (with a little less milk) and add clear strawberry flavoring made by Magic Line to taste.
Make regular buttercream, but for the liquid, substitute the "juice/syrup" from frozen strawberries
I made my regular BC recipe, and then I added strawberry syrup to it to taste. The kids loved it! It was the Hersey's strawberry syrup that you add to milk.
I usually mix strawberry preserves into my regular buttercream, I like having the little chunks of strawberry mixed in.
I make an "old fashioned" strawberry buttercream which is just butter, powdered sugar and fresh strawberries however it is perishable and it gets weepy if you leave it out of the fridge too long. It's mighty tasty though.