Film posters do it, sports team kits do it, and painters have been doing it for centuries: put orange next to blue, or red next to green, and each colour looks more intense than it does anywhere else. Artists call this pairing "complementary" and place them opposite each other on the colour wheel. It's taught as a rule of composition, which makes it easy to file away as a stylistic convention — a thing painters decided, the way they decided the rule of thirds. It isn't only that. Part of why the contrast works is sitting in your retina before the image ever reaches anything you'd call taste.
The eye already sorts colour into opposing pairs
Human colour vision doesn't send red, green and blue signals to the brain as three independent channels that get compared later. Cells further along the visual pathway recombine those three raw signals into two opponent channels: one that reports red-versus-green, another that reports blue-versus-yellow, plus a third for light-versus-dark. Each channel can only report one side of its pairing at a time — a signal can't be "reddish-green," because red and green are wired as opposites competing for the same channel, not as separate dials. This is opponent-process theory, and it was proposed on the strength of exactly the kind of everyday effect that gives it away: stare at a saturated patch of red for thirty seconds and look at a blank wall, and you'll see a green afterimage. The red channel that had been firing hard gets briefly exhausted, and the opposing green signal, unopposed for a moment, comes through on its own.
Placing true opposites side by side pushes that same machinery in the other direction, while it's still fresh rather than fatigued. Orange sits partway between red and yellow; blue sits opposite that mixture on the wheel. Set them against each other and the channels reporting each colour are being driven toward opposite extremes simultaneously, which is most of why the pairing reads as more vivid than either colour does against a neutral grey. The canvas isn't tricking the eye into an illusion it wouldn't otherwise have. It's giving the eye's own opponent wiring the clearest possible signal to report.
Why the wheel itself is a choice, not a law
The colour wheel art classes teach — red, yellow and blue as primaries, with orange, green and purple as the mixes between them — is a pigment-mixing model built for painters, not a description of how light physically works. Mixing coloured light follows different rules than mixing coloured pigment, and modern colour science uses more precise models than either. All of them still land on some version of "opposite hues intensify each other," but exactly which hue counts as which other hue's opposite shifts a little depending on which model you're using.
What we're still unsure about
Opponent-process theory explains why staring at a colour produces its opposite as an afterimage, and it's a solid physiological basis for why complementary pairs read as more vivid together. It doesn't fully explain why a specific pairing feels emotionally "warm and cool" or "tense and calming" to a viewer — that layer is much more clearly shaped by cultural association and individual context than the underlying retinal mechanism is, and treating the emotional reading as equally hard-wired would be overreaching past what the vision science actually supports.
This sits inside Colour Theory & the Colour Wheel, one of seven topics in Painting, one of five domains in Art, one of seventeen subjects the app can quiz you on.