I love colorful foliage plants. They also are perennials and would come up here after year. I think this is a beautiful idea to plant these hosta type plants under a tree with all their different colors
Allium 'Globemaster' - Alliums are nicknamed "Ornamental Onions." - The flowers (which are spheres formed of a whole lot of tightly packed tiny flowers called florets) can be softball to volleyball sized on stems up to three feet tall.
Hydrangea paniculata Limelight, large white blooms tinged with pale green; trailing Ivy at the base spills over the sides of the classy zinc planter boxes. Baltic Ivy (variegated creamy edge) would combine well too.