The blue and black stripes made white: the “yellow” circle is white.
Tricks of color
No yellow
There are three circles: cyan, magenta and yellow. Is the yellow one really yellow?
No: there is no yellow anywhere in the picture. Inside that circle the stripes are only white and black.
Why? Everywhere else, the blue stripes look like a blue screen in front of the picture. Inside the circle the blue is missing, and your brain reads “white with the blue taken away”, which is yellow.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)