It is known that the visual system resolves shape ambiguity into one of several stable states—consider the famous rabbit-duck drawing, or the Necker cube (a simple line drawing of a 3-D cube). In each case, the brain makes a categorical decision about what is represented in the image (the Necker cube is either popping out or receding). While multi-stable shape illusions abound, multi-stability has never been observed for color perception. Color is notoriously ambiguous—many color illusions exist—but until now it has been thought that all people with normal color vision experience color illusions the same way.  How does the visual system resolve color ambiguity? In this talk I will discuss work that addressed this question by quantifying people’s perception of a particularly ambiguous image, ‘the dress’ photograph. The colors of the individual pixels in the photograph when viewed in isolation are light-blue or brown, but popular accounts suggest the dress appears either white/gold or blue/black. The disagreement about the dress color contradicts our intuition that color is an unequivocal, stable, physical property of the world, and led to one of the most widespread internet sensations in history. The phenomenon suggested a major scientific discovery: the either/or nature of people’s perception suggested that color can be subject to multistability.  But there is a less exciting alternative hypothesis, more probable given present knowledge of color vision: that the opposing camps arose because the social-media question was framed as a two-alternative forced choice, with options falling on either side of the truth. (Blue and white fall on either side of light-blue; gold and black fall on either side of brown.)  Perhaps all people see light-blue/brown, but report a best guess given two imperfect options.  We tested over 1400 people, both on-line and under controlled laboratory conditions. Subjects first completed the sentence: “this is a ___and___dress”. Then they performed a color-matching experiment that did not depend on language. Surprisingly, the results uncovered three groups of subjects: white/gold observers, blue/black observers and blue/brown observers. The findings, recently published in Current Biology (DOI: 10.1016/j.cub.2015.04.053), document one of the most compelling examples of individual differences in color perception, and show that the brain resolves ambiguity in ‘the dress’ into one of three stable states, not unlike how the brain handles ambiguities in shape. But these states seem to be stickier than they are for shape. Only a minority of people switched which colors they saw (~11%).  It is clear that what we see depends on both retinal stimulation and internal knowledge about the world. Cases of multi-stability such as ‘the dress’ provide a rare opportunity to investigate this interplay.  In particular, we go on to demonstrate that ‘the dress’ photograph can be used as a tool to discover that skin reflectance is a particularly important implicit cue used by the brain to estimate the color of the light source, to resolve color ambiguity, shedding light on the role of high-level cues in color perception.