You see a faint glowing circle of color filling the space between the colored segments, like a neon light behind frosted glass. Only the thin lines are colored; the white between them is untouched.
Try this
- Change the Color of the inner segments and the glow follows it.
- Tick Remove the black lines: the glow mostly disappears.
Why it happens
The colored segments continue black lines, and that combination makes your visual system treat the colored part as a transparent colored layer lying over the grid. It then spreads the color into the white gaps to fill in that layer, and draws a soft edge where the black lines start. Harrie van Tuijl described this "neonlike color spreading" in 1975. Without the black lines the colored segments are just lines, which is why the glow fades.
Did you know?
The effect works with almost any color, and it is strongest when the colored segments are thin and continue the black lines exactly.
References
- van Tuijl H. (1975). A new visual illusion: Neonlike color spreading and complementary color induction between subjective contours. Acta Psychologica, 39, 441-445. doi:10.1016/0001-6918(75)90042-6
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Is the white between the lines colored?
No. Only the line segments are colored.
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