Kevin is definitely the scab expert, so I’m sure he will have a great answer for this!
I think it’s fascinating how blood changes color, and I can tell you a little bit about how blood in our veins changes color. There’s a cool molecule in the protein hemoglobin that makes red blood cells red, it’s called heme. It has iron in it and will change from bright red (when attached to oxygen) to darker purple-red (when oxygen is not attached). When these red blood cells die, the heme is released and will be broken down into different compounds that are different colors- it becomes biliverdin & bilirubin, which are greenish-yellow in color. In some people that have liver problems, their serum (blood without clotting factors or red blood cells) is greenish-yellow (normal serum is clear-tan). Babies sometimes have yellow skin and eyes (jaundice) because they haven’t quite grown enough of their liver to process these compounds. Eventually they are broken down all the way and we excrete them.