It is mostly masking fluid to preserve the white gaps between the leaves, then lifting the color around the edges to make it appear soft and hazy. It should be a color that lift easily, such as yellow ochre. These leaves were mostly done by painting in yellow ochre, then dropping in dry into wet various greens and browns.
No, its not supposed to stay white either, its supposed to yellow so you could see it and remove it.
Masking fluid is the first thing you paint, when you know in advance which areas you want to remain white on your drawing. ( flowers for example, railings, anything that will have a highlight and so forth) . You rub it off with your fingers after you have painted and youre left with white of the paper.