1. How to interpret "the tree" on Sethe's back?

The author mentions "the tree" , which refers to the scar on Sethe's back,  several times in the book. The description of trees can be found in the very beginning of this novel when Sethe recalled a situation that a fugitive nigger had been hung by his slaveholders. Then, we know that Sixo was tied up on a tree and burned to death after being captured from his escape. "The tree" here is not a real tree but a metaphor of Sethe's scar on her back, or I think we can also consider it as a symbol of black people's persecution since their misfortunes are often related to trees. According to a white girl, the shape of the scar on Sethe's back looks like a chokecherry tree. But to my understanding, it's more like a proof of the slavery which is not only a spiritual wound but a healed wound haunted her for the entire life.

2. What do "Boston and velvet" stand for in the book?

The "Boston and velvet" came from a girl whose name is Amy. Sethe met the girl while escaping form Sweet Home. By then, the famished pregnant Sethe learned about that Amy was heading to Boston to get some velvet. Boston, velvet, these words had never appeared in Sethe's life. Without giving a hand to Sethe, in the journey of Sethe's crawling, Amy walked along side her and talked much about velvet and Boston that somehow urged Sethe along and made her get through the crawling distance. To Sethe, Boston and velvet stand for a hope that she'll never approach to but a temptation that could drag her proceed to the lean-to house. To me, "Boston and velvet" as objects also represent the disparity between the class of white and black.

3. Why does the author mention the color in a quilt for two times?

I agree with Rebecca's explanation on this topic since I really don't know how to answer this one properly.