These color differences do not indicate that the meat is spoiled or old. Fresh cut meat is purplish in color. Oxygen from the air reacts with meat pigments to form a bright red color usually seen on the surface of ground beef purchased in the supermarket. The interior of the meat may be grayish-brown due to the lack of oxygen penetrating below the surface.

Additional Resources:

http://www.fsis.usda.gov/help/FAQs_Hotline_Appearance/index.asp#3