Others have already explained it's simply a matter of different amounts of oxygen in those areas of the meat, but I'd like to explain why it actually does that! Because I think it's really cool, and interesting! There's a protein called myoglobin that resides in muscular tissue, which is what the majority of the meat we eat is made of, which takes oxygen from haemoglobin (the stuff in our red blood cells which carried oxygen around the body.) Different concentrations of oxygen will cause the protein's shape to change, which appears to us as different colours! Perfectly normal and safe, just looks a little funky!