This is not a green square. It’s a grid of green squares, separated by little black lines.
Each green square represents the proportion of privately-owned guns in America not used in a homicide.
The red square represents the proportion of privately-owned guns that are used in homicides. 
Americans legally own close to 300 million guns; roughly 8,000 a year are used in homicides. That’s about one quarter of one percent of one percent.
You could show a similar image with the green squares representing total annual deaths and the red squares representing annual deaths due to terrorism.
Just sayin’.
False equivalence, lunchbox.