Why does bill moyers ask alice walker, "where do you see at this moment the color purple? "

Alice Walker described purple as a representation of something that's rare but is there. Bill Moyers wanted to know what rare thing she sees at the moment and she eventually gave the kindness of Bill Moyers as an answer. 

Sometimes when you read things in your head, you skip overall small details/words. when you read aloud, you hear those words and it’s easier to comprehend what you just read. saying things aloud also you remember them.