Two thin wavy lines — one dark, one light — and your brain fills in an entire region with color that isn't there.
What: The Watercolor Illusion is one of those gorgeous ones where your brain is just… painting things that aren’t there. The thing about this thats beautiful, is that it’s easily paintable (water colors obviously) with your own watercolors, on physical paper.
You should effectively be able to look at this illusion and figure it out (it seems to work for me in greys/blacks/whites so its color blind-independent?) … the lines drawing the shapes have a subtle shading to them in one direction, not too dissimilar to water color painting. those shades only got a small distance, but your brain will decide to “fill” the remaining space, creating a layering effect. But it’s white, trust me.
It was discovered by Baingio Pinna back in 1987 [1], though he first wrote about it in Italian so the rest of the world didn’t really catch on until he published with Brelstaff and Spillmann in 2001 [2]. That paper gave it the name “watercolor illusion”, again, self explainitary and perfect.
How To: The inside of those shapes should look faintly tinted, orangey, peachy, like someone breathed color onto it. It’s not there. Grab a color picker or zoom way in if you don’t believe me. I’ll wait.
Click anywhere to flip which direction the color spreads and you’ll see the outside “fills” instead of the inside, which is wild. In the controls you can swap the inner and outer colors around, crank the wave frequency up (higher frequency = stronger effect), or thin out the lines.
Extra cool feature alert: Toggle on Draw Mode and you can sketch your own shapes to see the effect on anything you like. The shading should invert as you draw shapes in shapes, so a bullseye can be especially cool.
Explain it: The key is luminance contrast asymmetry. The outer line is dark (high contrast against the white background), the inner line is light (low contrast). Your visual system sees that soft boundary and just… keeps going, filling the whole enclosed area with color that spreads from the low contrast edge inward.
Complementary color pairs work best, orange and purple is the classic combo, and it’s what Pinna used in most of his demos. Break the luminance asymmetry (make both lines the same contrast) and the illusion basically dies. Your brain needs that mismatch to get confused.
As always (it seems) the jury is a little out on the science. The best models involve interactions between boundary processing and feature processing across various visual cortex [3]… but it’s one of those things where we know what happens way better than why it happens.
Fun fact: Jack Broerse and Robert O’Shea independently discovered the same effect in 1995, calling it “spread colour.” Two groups, eight years apart, stumbling onto the same trick your brain plays. Even dotted or dashed lines can produce the effect, though continuous wiggly lines like the ones here are strongest.
I've researched these optical illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via toymaker@toms.toys, be kind!