It looks to me like a thin layer of very soft buttercream on a regular wide icing spatula, added while you are rotating the cake on a turntable.
Light shade at top is of course the base icing. I would hold the spatula upright for the middle shade. Then another layer at the bottom using the spatula sideways to control the width of that shade. The a last pass with the spatula to smooth it all out.
Your shading buttercream has to be perfectly smooth and a little soft. If you have to, thin your buttercream with a tiny bit of corn syrup to make it spread easily. And I think that using a petal tip would add too much thickness.