This appears to be a bruise which is healing. Bruises while healing undergo various stages with color differentiation. Initially at the point of the trauma the wound will be pink and red which continues to becomes blue and black within a day, around the 6th day this color will change to green and from the 7th day it becomes yellow and brown which is the last phase of the healing. You can speed up the healing by applying an ice pack over the area which will help. If the lesion remains persistent or worsens visit the ER at the earliest.

Hi there. It's just a bruise caused due to trauma which you may have not noticed. Since breast contains more fat tissue, even slightest of trauma could cause bruise. Bruise has changes from red to slight blue hue to green to yellow. In another 2-3days this yellow would disappear even.