Only the lightness kept, as a black-and-white camera sees it: the squares disappear into the gray.
Tricks of color
Helmholtz–Kohlrausch effect
Which looks lighter, the colored squares or the gray around them?
They are all exactly as light as the gray, but the colors look lighter.
Why? Strong colors look brighter than they measure. It is why a red or purple light looks so bright, and why photos lose some of their glow in black and white.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)