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Why do some colors look special? Color vision is our ability to distinguish the spectral characteristics of light. Wavelength varies continuously, yet we perceive spectra in terms of a small number of dimensions anchored by salient landmarks. These dimensions define a color space, within which colors appear to vary in hue (direction) or saturation (distance) relative to a neutral gray (Figure 1). Coding relative to a norm may reflect a general strategy in perception. For example, individual faces may be judged by how they differ from a neutral or average face. Any hue can be decomposed into a mixture of the perceptually pure hues red, green, blue or yellow (for example, orange looks like a combination of red and yellow). However, no light looks both red and green, or both blue and yellow, suggesting that these hues reflect opposite poles of two opponent dimensions, in the same way that light and dark are opposing aspects of brightness. Conventional models of color vision assume that perceptual opponency is mirrored by neural opponency, and that the primary hues look unique because they represent unique states in the neural code. For example, a ‘red versus green’ cell might be inhibited by one type of cone receptor while excited by another (Figure 1), and we might see pure blue or yellow at the wavelengths that precisely balance or null the antagonistic inputs to the red–green mechanisms. Color opponency in single cells is well established. Yet the tuning characteristics of these cells do not match the red–green and blue–yellow dimensions of color appearance. This has raised the possibility that the reason some colors look special is not because of how we are built, but because of how the world is built. For example, blue and yellow may look prominent because they describe how daylight Quick guide been at a useful scale for scientific studies, bathymetry experts say that the public’s ability to access such information about the oceans will allow them to gain a better understanding and will impact on their perceptions of this major part of our planet.