Paul Riley
The reason negative painting is peculiar to watercolour painting is that traditionally white paint isn’t used for white or light objects. That method is reserved for opaque media such as oils, acrylics and pastels. The whole point of watercolour painting is its transparency and translucency – watercolour paint works by allowing light to pass through the particles of pigment, the colour reflecting from the white of the paper – thus any opacity will affect that quality and make the painting appear dull.
In other words, the paper is in fact your white paint. To make a lighter colour, you dilute the paint with water, which is why the water needs to be clean. If you want a completely white object you leave the white paper unpainted; the surrounding colour tone reveals the white object – you paint the negative shape, hence the term negative painting. Simple, but how do you know what shape it is if it is white on a white surface? You need to indicate it, and this can be achieved in various ways.