John Good is the witness whose testimony is being discussed here. He said it was so dark, he could not tell the ethnicity of either person, just that they both looked and sounded male. He yelled at them to "cut it out" or some such, and Good says the man on top yelled, "Call the police!" to him.
Afterward, he was informed by police that Martin was 6 feet tall, Zimmerman 5 feet 8 inches. Four inch difference, which is a lot if both are standing near each other...but nope, Good saw the straddling man throwing punches at the supine man. How the heck can you be sure which man is the taller?
Also, since Good established it was so dark that he could not tell skin color differences, how could he be sure about jacket/hoodie color? Clothing color is the number one thing that eye witnesses get wrong nearly 90% of the time in good lighting. People focus on the startling actions, not the clothes; stress fucks up recall ability. Police academies drill on initial clothing ID recollection more than any other factor in trying to train cadets to become reliable eye witnesses.