In the natural world, nearly all colors that we experience are seen within a context of surrounding light and nearby objects, and this context strongly affects color appearance. In this way, one of the most important mechanisms in color appearance is color assimilation. Color assimilation is a shift in the perceived color of a region toward the chromatic appearance of a nearby inducing stimulus. The Watercolor effect is a new interesting phenomenon for studying such process. Here, color assimilation occurs within an area enclosed by a light chromatic contour (e.g., orange) which in turn is surrounded by a darker chromatic contour (e.g., purple), thus the central area will appear orangish. At the core of the project is the fine investigation of this visual phenomenon. How chromatic assimilation is integrated to generate perceived colors is a cornerstone in color vision. Here, some experiments using psychophysical and fMRI methods are proposed to elucidate such questions. As a result, relatively little is known about color assimilation. Indeed, most studies showed a diversity of patterns producing color or brightness assimilation but quantitative measurements were not systematically reported. Contour parameters producing the coloration effect have been recognized from some previous work and the importance of these changes has recently been emphasized. Data are still lacking, however, that identify specific conditions responsible for color assimilation. To that end, some set of experiments are proposed to determine the strength of the effect depending on contour parameters (widths, waviness and separation) but also as depending of background luminance. Our originality is to determine precisely how parameter modulations change our perception of color using some psychophysical techniques. Thus, observers will report in a cone-based color-space what they perceived precisely using a hue-cancellation method. A set of experiment will be also developed to propose an explanation of this phenomenon. The coloration effect is characterized by a spread of color from the inner contour onto the enclosed surface area, suggesting a global effect from sparse (local) stimulation. The question is how the color diffuses out of the boundary to fill the adjacent area. These phenomena can in principle be explained by two alternative theories. One assumes that color and brightness signals actually spread from the borders into uniform regions at the same cortical level. Other postulates that color is represented at a higher level as attributes of object surfaces, and assumes that the phenomenal color spreading corresponds to a change in this abstract representation. Here, we propose to determine the locus of neural activity of color assimilation and to test these competing hypotheses using an fMRI method. We will also respond to the following question: Does the visual system respond to physically uniform surfaces in the same way as color assimilation? Finally, we will extend previous experiments by characterizing the relationship between parametric variations in color assimilation phenomenon with cortical responses. Moreover, we will display and analyze the results as a function of the retinotopic visual areas. The retinotopic procedure will distinguish the fMRI responses obtained from the contours to those obtained from the inner surface. The proposed experiments will provide an important step to understand the color appearance mechanisms recording behavioral data and by the way of neural correlates of color assimilation that never been explored.