The Department of Agriculture explains here that any ground beef can naturally be brown on the inside. The pigment found in all red meat, known as oxymogloin, combines with the oxygen in the air to produce the external red color associated with fresh beef. This process is called bloom.
The interior of the meat does not have the red color due to the lack of oxygen necessary for this reaction to occur. Stephanie, one of our food safety experts, explains this process in her blog which you can read by clicking here. Rest assured that the brownish colored beef in the middle of your ground meat is just as juicy and flavorful as its bright red exterior.