We tend to think of colors as fixed and universal, but that’s not the case across cultures and populations. The Japanese, for example, see what we would identify as a green stoplight as being a shade of blue. Yellow, however, appears to be one color that is stable, and researchers from the University of York in the U.K. wanted to find out why. As it turns out, our bodies adjust the way we see yellow depending on the season in order to keep it consistent.

Color, or the degree to which we perceive it, is highly contextual – people with darker skin are often perceived as having very white teeth regardless of their actual hue, for instance. For humans, all of the colors we see are mixtures of four unique hues (that is, colors that do not appear to be combinations of others) – red, yellow, green and blue. The researchers hypothesized that yellow’s stability might be owed to the world around us, and not biology.

67 men and women were tested once in June and once in July. After sitting in a darkened room, they were allowed to adjust to the light and were then placed in front of a machine that showed an adjustable colored image. The subjects were asked to tune the color balance until the screen showed what they thought was a pure yellow, with no hint of green or red.

The researchers found that in the summer when there’s more foliage, our eyes adjust for all that extra green so that things are still perceived as “pure” yellow.

“In York, you typically have grey, dull winters and then in summer you have greenery everywhere. Our vision compensates for those changes and that, surprisingly, changes what we think ‘yellow’  looks like. It’s a bit like changing the colour balance on your TV,” said lead author Lauren Welbourne.

Basically, while it’s easier for things to stick out as yellow in a grey, lifeless winter landscape, our eyes develop higher standards for yellow when there’s more competing green. This adaptation may have been important, as brightly colored yellow things are often either valuable (fruits) or dangerous (venomous animals and insects). Being able to identify them was likely important for survival.

The researchers say that the findings don’t help treat any particular disorder just yet, but that it’s an interesting discovery that sheds new light on the way that humans perceive, well, light.