Figure 2 Watercolor effect test images. The revised version of the watercolor illusion based on the original concept of Pinna et al [16], adapted under CC BY-NC-SA 3.0. Purple undulated contours adjacent to orange ones are perceived as evenly colored by a light veil of orange tint spreading from the orange contours (coloration effect); (A) 4 large squares, each containing smaller squares at their center, with 2 conditions: 2 squares where WE was visible between 2 orange lines, extending from the inner part of the larger square to the outer part of the smaller square, and 2 squares where WE was visible within a smaller square; (B) a large rectangle with 2 smaller rectangles inside, where WE was visible between the 2 orange lines, extending from the inner part of the larger square to the outer parts of the smaller squares. Created using Microsoft Paint, version 6.3 (Microsoft Corporation, Redmond, WA, USA) and Microsoft PowerPoint, Microsoft Office Professional Plus 2016, version 16.0.18827.20102 (Microsoft Corporation, Redmond, WA, USA).