This image is an example of visual design of color and shape.
All the circles are in red, while all the semicircles are in magenta.
All the triangles are in two different kinds of green, and squares are in gray/brown.
To me, the first poping-out objects in this image are the two shapes in blue, and the second are the red circles.
It makes sense according to the feature channels and hierarchies.
The blue is the brightest color in this image, while the shape of circle is one of the most pop-out shape in general.