When you paint with oils, you sometimes have wonderful "accidents." In painting my favorite pear trees, I first applied a purple transparent layer (glaze) in some areas. It smeared around a bit and started to spread. I liked how it looked so I continued applying transparent layers of different purples hues over the whole tree area. Then I applied the various whites and pastels, pink and yellow, on top of it. Voila! My best "white" painting ever. It is usually hard to make an interesting white painting, but this somewhat painted itself.