When Lady Macbeth dies, he feels nothing, he is emotionless. That might tell us that she has turned him into a ruthless killer, who will stop at nothing to get what he wants, and what he wants is the kingdom. He is very calm, as if he doesn't care at all that his wife died - she has turned him into a mad monster. He has lost all ability to feel, at least until he finishes what he has started.
Adjectives are descriptive words that describe other words and answer a question.
Thus, in your sentence, the adjective is the word red, as is tells the reader what color the shirt is.
