science · FOLLOW YOUR CURIOSITY
Can the same grey look like two different colours?
Two identical squares. Two different surroundings. Your eyes compare more than the patch you are looking at.
Explore the interactive discovery →
Start with what you see
Place identical grey patches on a light and a dark background. The patch on the dark field often looks brighter. The patch has not changed: its surroundings alter its appearance.
What is happening?
This is simultaneous brightness contrast. Visual processing responds to relationships between neighbouring regions, not just isolated light levels. The demonstration shows an effect of context; it does not diagnose your vision or explain every aspect of colour perception.
A closer look
Look at the centre squares, then remove their different backgrounds.
Both centre squares use exactly the same colour. Only the background changes when you press the button. Screen settings and viewing conditions can influence the strength of the effect.
Try a concrete example
Before pressing the control, predict whether the two centre patches will change together or stay identical. Then compare them with the backgrounds equalized. The demonstration separates the stimulus drawn on the screen from your experience of its brightness. It does not mean the eyes are defective, or that every colour judgment is arbitrary. It shows why comparisons are easier when lighting and surroundings are controlled, and why a colour swatch can seem different when placed in a new setting.
AN IDEA TO TAKE WITH YOU
What you see depends partly on what surrounds it.
Follow a connection
Why is the sky blue if sunlight looks white? →
Perception changes the grey you see. In the sky, scattering changes the light that reaches you.
Sources and transparency
Vision Research · Mechanisms Underlying Simultaneous Brightness Contrast
Accessed: 2026-10-31
New York University · Brightness and contrast — David Heeger
Accessed: 2026-10-31
Illustrations explain an idea. Sources are available above.