A simple correspondence between S-cone response and the sensation of blueness has long been questioned. We measured color appearance using a series of S-cone isolating stimuli and a paired comparison paradigm to identify a direct correspondence between S-cone response and perceived hue strength. The results revealed a non-monotonic reversal in perceived blueness at the unique-blue line: The S-cone increment initially increased blueness, but as S-cone excitation grew past the pure-blue line, blueness decreased and redness increased. It is difficult to account for this pattern of results by supposing that the sensation of blueness is mediated by a conventional linear channel that extracts (S+M)/L: If the ratio L:M is held constant, the output of such a channel should always increase with increasing S, and so inhibition from a second channel would be needed to account for our result. We also consider another category of explanation: The appearance of a given chromaticity may depend on its position relative to the fundamental axis of daylight locus, which serves as an internalized reference. In an account of the latter kind, there is a systematic mapping between chromaticities and sensations, but there does not need to be a distinct neural signal that represents blueness. (c) 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 323-328, 2015