WHAT is a bruise?
A bruise, or contusion, forms after a hard-enough bump to the soft tissues under your skin.
When these tissues are injured small veins and capillaries break, leading to red blood cells leaking out. These red blood cells collect under your skin causing the bluish, purplish, reddish or blackish mark.
First, you will probably have a bump that looks red and feels tender.
After a couple of days the bruise will start to look blue, sometimes even black.
After about a week it might look greenish or even yellow.
After two weeks, the bruise will be a light brown. - faqkids.com; kidshealth.org; medicinenet.com