The watercolour illusion exhibits two remarkable phenomena: firstly, there is a long-range ‘spreading’ of a pale colour from one or more brightly coloured contours into the surrounding space; secondly, there is a pronounced figure-ground effect, in which certain elements will be perceived as having a definite shape (the figure) while others are perceived as a formless ground which is being occluded by the figure. In the main image on this page, the space inside the two smaller contours appears as the ground, which we glimpse through gaps in the figure bounded by the larger contour. Note that reversing the juxtaposed colours will cause both the colour-spreading and the figure-ground relation to also reverse: see figure 1.​Figure 1 – Reversal of colours causes reversal of figure and ground

The figure-ground effect in the watercolour illusion is powerful; it defies many of the Gestalt principles which were formulated by the Gestalt school of psychology which emerged in early 20th century Berlin. ‘Gestalt’ roughly translates as ‘form’ or ‘organized whole’ – the Gestaltists sought to uncover the principles of perceptual organization which determine how we perceive objects, groups, and relational properties (see Todorovic 2008 for further discussion). Gestaltism embodies a kind of ‘mid-level’ approach to perception, as opposed to low-level retinal/neural explanations and high-level cognitive/reasoning-based explanations. The figure-ground perceptual experience produced by the watercolour illusion defies Gestalt principles like continuity, closure, symmetry, past experience, and Prägnanz (Pinna 2008; Prägnanz is characteristic of Gestaltism, and means a kind of parsimonious interpretation of stimulus). Gestalt psychology’s popularity has waxed and waned over the years (see Wagemans et al. 2012), not least of all due to its methods, which drew greatly on phenomenology or introspection on the subjective character of experience. However, perceptual organization remains an area of active research; philosopher Tyler Burge discusses Gestaltist concepts as an attempt to explain our objectified representations of the world based on sensory stimulus (Burge 2010).

The coloration effect seen in the watercolour illusion is similar to that of the neon colour spreading illusion, which is often explained with reference to boundary grouping and perceptual filling-in. Figure-ground effects may also be explained by these same processes - see Grossberg (2015).

Boundary grouping is the process or processes in perception that determine what is taken to be all the one boundary or all the one contour of an object ( as opposed to different contours of different objects). This is often not a straightforward task as parts of the countour of an object may be occluded, some parts may be nearer or further away from the perceiver, and so on.

The concept or concepts of ‘filling-in’ are explored in the entry for the Troxler Effect.