This painting is the final one of Dianna Shyne’s “Cloud Challenge” at Acrylic University. It’s called “Silver Lining”.  The sky is a mix of Cerulean Blue (PB35), Ultramarine Blue (PB29) and white. I find the Cerulean Blue (at least by this brand — Charvin — to be quite transparent, which was unexpected. 6×6 canvas panel.