Can Scent Change What We Judge to Be Gray?

A cherry scent may bring red or pink to mind. But if you are trying to create ‘gray with neither redness nor yellowness’, rather than choosing a color that suits a scent, can the surrounding odor still enter your judgment?

In 2023, Ryan J. Ward and colleagues asked participants to adjust an on-screen color until it appeared achromatic in a scented room. The aim was to test whether odor can influence color appearance or judgment, reversing the familiar direction in which color changes impressions of a smell.

Creating a hue-free gray, not a color that matches the scent

The 24 participants were aged 20–57, with 13 women and 11 men. They reported no problems with smell or color vision. Color vision was assessed by self-report, not by a test before participation.

Five scents were used: caramel, cherry, coffee, lemon, and peppermint. Water without added fragrance served as the control. Fragrances were added to water and dispersed through the room using an ultrasonic diffuser, with odor order randomized. The device was positioned out of participants’ sight.

Participants adjusted a square color patch on a screen in a darkened room. Two sliders changed the red–green and yellow–blue directions, and they selected the point that seemed to have neither hue. Brightness was fixed; only the chromatic component could be freely adjusted.

The task was not to select a color that matched each scent, but to create a patch that looked gray. Odor names were not supplied beforehand. After choosing the color, participants selected what they thought the odor was from a list of options.

The researchers expected compensation in the opposite direction

The team expected the color patch to be adjusted away from the color associated with the odor. For example, if lemon scent induced an impression of yellow, participants might add a little blue to the patch to feel they had removed its hue.

The average adjustments did not follow that prediction. Relative to the unscented condition, the five odor conditions generally shifted toward warmer colors containing yellow or red. The changes reported in color coordinates were less than one unit, rather than large shifts.

Mean settings shifted toward reddish brown with cherry and coffee, yellowish brown with caramel, and yellow-green with lemon. The authors interpreted four of these shifts as broadly corresponding to color directions previously associated with each odor. Peppermint shifted toward reddish brown rather than the green or blue commonly associated with it.

That is not the same as saying ‘gray looked red’

The crucial point is that the recorded measure was the on-screen color participants selected because it looked gray. Even if they chose a slightly reddish setting, they judged that setting to be hue-free.

The finding therefore cannot simply be restated as ‘cherry scent makes gray look red’. It shows a small change, relative to the unscented condition, in the color setting accepted as achromatic. The experiment did not show objects around a room appearing dramatically different in color.

The evidence was not equally strong for all five scents

A warmward tendency in mean values is different from consistency in shift direction across participants. Tests of whether differences from the unscented condition had nonrandom directions found significant biases for cherry, coffee, and peppermint. Caramel and lemon did not meet this criterion.

The original paper includes other comparisons, but interprets nonsignificant results in some directional tests as evidence of ‘different directions’. Nonsignificance does not prove a difference. Rather than concluding that the same kind of effect was established for all five scents, the results should be understood as small odor-related adjustment biases whose consistency varied across odors.

Can familiarity matter even without knowing the name?

Correct odor identification was about 33% each for caramel, cherry, and peppermint, and about 12% each for coffee and lemon. Clearly naming the odors was not easy for participants.

The authors consider whether knowledge of an odor’s identity or a sense of having smelled it before could influence color judgments. Even when a name cannot be recalled, experience-based associations with color might still operate.

When an odor is not recognized, they suggest that intensity, pleasantness, irritation, or odor-evoked emotion might instead contribute. They also discuss an alternative explanation: participants may have adjusted the color toward a pleasantly balanced odor–color combination. These data alone cannot distinguish among those mechanisms.

Because the bias arose in a gray-adjustment task that did not use odor names, the authors emphasize a possible influence on perception itself rather than merely verbally induced color associations. However, the experiment did not directly separate the decision process involved in selecting an answer from the process of seeing color.

Testing brightness and odor familiarity too

The brightness of the color patch was fixed in this experiment. Earlier work has reported relationships between odor intensity and the brightness of associated colors, so allowing brightness to vary could produce different results.

The study also involved only 24 people and five odors. More participants and odors are needed to test whether similar patterns occur for readily named and unfamiliar smells. Including odors associated with blue and green would help distinguish an overall warmward bias from associations specific to individual odors.

The authors declared no commercial or financial conflicts of interest related to the research.

Summary

The ‘hue-free gray’ selected in scented rooms shifted slightly from the unscented condition, toward warmer colors on average. The researchers’ predicted explanation, compensating for the color associated with an odor by adding its opponent color, was not supported.

What makes this study interesting is that scent can enter a task of deciding what looks gray even when participants are not asked which color suits an odor. Testing whether this arises from knowledge about smells, emotion, or altered perception could clarify at what stage information from the eyes and nose comes together.

References

Ward, R. J., Ashraf, M., Wuerger, S., & Marshall, A. (2023). Odors modulate color appearance. Frontiers in Psychology, 14, 1175703. DOI: 10.3389/fpsyg.2023.1175703. Full original article. © 2023 Ward, Ashraf, Wuerger and Marshall. The original article is published under CC BY.