Lesson
Black as coal: extremely dark or evil. His heart is black as coal
Blue: Depression .I feel blue to day
Gray area: unclear and uncertain issue. I'm having trouble answering you because that's a gray area.
Green: novice. I wouldn't hire him if I were you, he's really green
Red: (derogatory) communist. I think he's a Red. Notice this slang term was used in an American movie title in the mid-1980's called "Reds"