Cause .500 is a perfect light. Anything below that is a redlight. (aka foul start)
http://www.nhra.com/basics/dayatdrags.html
"Full Tree: used in Competition, Super Stock, and Stock, for which a handicap starting system is used to equalize competition. The three amber bulbs on the Christmas Tree flash consecutively five-tenths of a second apart, followed five-tenths later by the green starting light. A perfect reaction time on a full Tree is .500."
http://www.nhra.com/basics/dayatdrags.html
"Full Tree: used in Competition, Super Stock, and Stock, for which a handicap starting system is used to equalize competition. The three amber bulbs on the Christmas Tree flash consecutively five-tenths of a second apart, followed five-tenths later by the green starting light. A perfect reaction time on a full Tree is .500."

