NEGATIVE SPACE. What do you see? A green octagon or a black octagon?
There are a couple of features of the game that are designed to cause confusion. One of these is the use of negative space, i.e. an absence or something that isn’t there. Instead of seeing solid shapes, you’re looking at spaces created by shapes being removed. This causes confusion as your brain is trained to look for what is there, not what isn’t there. So the answer is you’re seeing a green octagon stencil on a black base.