Where might such beliefs come from? It would be hard to ascertain why someone assumes that something is in a shadow or not. In general, one can expect that people assume things to be more likely that they have encountered more often in their past in a Bayesian fashion. For instance, slower speeds are much more commonly encountered than faster speeds, so if someone is exposed to moving stimuli under observation conditions with some uncertainty, they will take into account that slower speeds are more likely than faster ones and downgrade their speed estimates accordingly (Stocker & Simoncelli, 2006 ). In the color domain, one would expect that someone who has encountered more short-wavelength light to assume a short-wavelength (or bluish) illumination if there is uncertainty about the nature of the illuminant. In contrast, someone who has encountered more long-wavelength lights in their past would be more likely to assume a long-wavelength (or yellowish) illuminant in the same situation. Those are specific predictions given access to someone's illumination prior, but how would one go about estimating someone's lifetime exposure to lights of different spectral content? One way to go about this is to assume that people who spend more time in daylight will spend more time being exposed to natural light whereas those who spend more waking time at night will be more exposed to artificial—until recently, usually incandescent—light. Fortuitously for us, there is quite some diversity in terms of the human propensity to get up at sunrise and go to bed at sunset (“larks”) versus those who are phase-shifted, getting up late and staying up later (“owls”). This circadian type is largely stable throughout the lifetime after around age 25 and seems to be strongly determined genetically (Gibertini, Graham, & Cook, 1999 ; Adan et al., 2012 ; Pegoraro et al., 2015 ). In addition, natural sunlight has different spectral properties than incandescent artificial light: The spectral power of incandescent light is shifted toward longer wavelengths (Smith, 2016 ). In other words, it seems reasonable to assume that larks have been exposed to more bluish sunlight than owls, who are exposed to more yellowish incandescent light. If so, we could use circadian type as a proxy for illumination priors and would predict that larks tend to see the dress stimulus as white and gold as they discount for the assumed bluish illumination whereas owls can be expected to see the dress stimulus as blue and black as they discount an assumed yellowish illuminant. With this rationale and these predictions in place, we asked the participants about their self-identified circadian type (lark = gets up early, goes to bed early, and feels best in the morning vs. owl = likes to sleep in and stay up and feels best at night).