Red = Bleeding
It either happens with every single hit or never depending on which version you’re playing.
You correct it with bandages or, in a pinch, rags.
Blue = Wound that’s going to infect you with the Zomzoms rotting Russian syndrome
Use first aid, disinfectant or the good old Lighter + Knife (any kind of flame with any kind of blade should work)
Green = Infected
You’re most likely screwed if you don’t have antibiotics on hand, refer to John’s post.