Sorry, but maybe it comes from the business term, "in the red"? :(
ANSWER:
Good question. I came across this on a sports blog:
"There isn't an official explanation from the NCAA or NFL on why the area from the 20 yard-line to the end zone is commonly referred to as the red zone. However, as the majority of the scoring takes place from inside the 20 yard-line, it is believed that the term came about because it is the hottest area of scoring on the field, and generates the most action. Sorry there wasn't a stranger or more exciting explanation. "
http://blogs.eveningsun.com/latter/qa/
Rugby sometimes uses the same term for the area inside the 22-meter line, so it could be that football borrowed it from rugby (as it did with many other terms).
· zone defense
In American football, 10 yards deep by 53 1/3 yards wide.
10 yards in American football; 20 yards in Canadian football.
Safety
Line Backer is an American football position. It is a defensive position.
limit of the zone of unttled land
Gator Football, also known as the Florida Gators, is an American football team. There are a few basic procedures for playing American football. The basic procedures are to outscore your opponent by advancing the football into their end zone for as many touchdowns as possible.
Zone defense is a sporting term in basketball, football and hockey.
In American football, a team gets 4 downs to reach either the first down marker or the end zone. In Canadian football, a team gets 3 downs to reach either the first down marker or the end zone.
Zone defense is a football term. It begins with the letter Z.
Zone defense is a football term. It begins with the letter z.
The end zone on an American Football field is always 10 yards (9.1 m) long.