A Trick of the Eye.
As you know in watercolour you cannot paint a light tone over a dark one, unless you use opaque paints.
When learning to paint we start by drawing a picture and then colouring-in the shapes, usually with paint straight from the tube or pan. This method doesn't allow for the beauty of watercolour to be fully exploited.
Negative painting is an alternative approach in which the subject is established by painting around the object, rather than by painting the object itself.