You bang your knee on the coffee table—ow!—and the next morning, there's a dark purple splotch blooming on your skin like someone dropped an inkblot under the surface. By the end of the week, it's faded to greenish-yellow, then vanished entirely. What just happened under there?

When you whack your knee, tiny blood vessels under your skin—capillaries, thinner than hairs—break open. Blood leaks out into the surrounding tissue like water from a burst pipe flooding a basement. Your body sees this mess and immediately sends in the cleanup crew.

Fresh blood is red because it's full of hemoglobin, the iron-rich molecule that carries oxygen in your red blood cells. That's why a brand-new bruise looks dark red or purple—it's just blood sitting where it doesn't usually belong, visible through your skin like ink through tissue paper.

Your body doesn't let spilled blood just sit there. Special cleanup cells called macrophages—imagine tiny vacuum cleaners—arrive at the scene and start breaking down the red blood cells, digesting the hemoglobin bit by bit. It's like dismantling a car for parts.

When macrophages break down hemoglobin, they split off the iron and convert what's left into a green compound called biliverdin. "Bili" means bile, "verdin" means green—so yes, "green bile stuff." This is when your bruise shifts from purple to that weird greenish shade, a few days after the injury.

But your body's not done. It converts the green biliverdin into another compound called bilirubin, which is yellow-orange—the same pigment that makes your pee pale yellow. Now your bruise looks like a faded banana skin, that sickly yellowish color that tells you it's almost healed.

Finally, the macrophages cart away the last scraps—iron gets recycled into new hemoglobin, bilirubin gets processed by your liver and kidnapped out—and fresh tissue fills in where the blood had pooled. The bruise vanishes completely, like someone erased the inkblot from the inside.

So a bruise isn't just sitting there changing color for fun—it's your body's demolition and recycling crew working overtime, breaking down trapped blood into green, then yellow, then nothing. The rainbow on your knee is a timeline of cleanup, and when it fades, the job is done.