Myoglobin and haemoglobin from the blood causes the red colour in meat. There is more myoglobin in well used muscle and in older animals this means the meat is darker. When oxygen is taken to the tissues the myoglobin is converted into oxymyoglobin, which is red. After slaughter the meat remains dark until it is cut and exposed to oxygen in the air when it becomes bright red.
If stored for a long time the meat may become green or brown due to the effect of enzymes and bacteria.
When meat is cooked the proteins denature making the meat firmer. The collagen is converted into gelatine in the presence of moisture. Elastin is softened slightly. Fat melts and penetrates the lean meat, increasing the energy value. The colour changes from red to brown.