Question: what cells form scabs and what changes the colours of them?

Matt Bawn answered on 7 Mar 2018:

They are a mixture of red blood cells, platelets, skin cells and soluble fibrinogen proteins that form a mesh of insoluble fibrin fibres across the wound . The red blood cells that get trapped in the fibrin mesh cause the initial red colour of the scab, due to the haemoglobin (body oxygen transport protein) inside the cell. The red blood cells will start to break down over time ans washing the wound begins to remove the haemoglobin The remaining empty dead red blood cells, platelets etc. lead to the brownish colour.