The orange and magenta taken away: both arms are the same green.
Tricks of color
Spiral of two greens
Two arms curl into the middle, a green one and a blue one. Are they?
No: there is no blue. Both arms are the very same green; only the stripes beside it change.
Why? Thin stripes blend into each other in your eye. Green beside orange takes some of the orange and looks yellow-green; the same green beside magenta takes some of the magenta and looks blue. David Novick drew the first, after the Munker illusion; this is our version.
Picture: Drawn by Science Playground, after David Novick · Wikipedia(opens in a new tab)