What optical illusion is this?

It demonstrates simultaneous (induced) contrast. Every grey circle on the board is exactly the same shade at any instant, but the human visual system judges brightness relative to the surrounding area, so a circle on a dark square looks lighter while the identical circle on a light square looks darker.

Are the circles really all the same colour?

Yes. All circles are filled with one identical grey value that pulses over time. Only the black or white square beneath each circle changes how light or dark it appears — the circles themselves never differ from one another.

You can change the number of columns and rows in the checkerboard, the flash speed of the circles, the crossfade speed of the board between black and white, and the amount of blur on the circle edges.

Induced Contrast Checkerboard — Interactive Optical Illusion

Mode

Columns

3

Rows

3

Flash

10

Board

2

Blur

3%