I read recently in Richard Schmid's book "Alla Prima" about using both thin paint (dark transparent) and thick (light opaque) paint to add dimension to a painting. James Gurney also recently describes using white as a thick impasto to make white sparkle as it sits above and casts a small shadow on the paint underneath. With that in mind, the closer umbrella has thicker paint than the one behind it. It seems to work to me.
You can check James Gurney's blog by clicking http://gurneyjourney.blogspot.com