The central pieces of each structure are the same color. This is an amazing optical illusion. If you want to check it, just copy and paste and use your Paint program. Exactly the same color.

Our perception is strongly determined by surrounding colours. Theres a lot of examples like this one, a classic:(BeepBeep version, drawn by me xD)

The spirals inside are made with the same grey, just try it yourself.

And this great drawing from one of my favourites artists, M.C. Escher, a real genius. I strongly suggest you to find more drawings by this exceptional man.