the color of pork changes gradually

Quick answer：

The discoloration of pork is a common phenomenon due to oxidation. Pork contains myoglobin, an iron-containing protein, which can turn into oxymyoglobin when exposed to oxygen. This causes the pork to change from its original red color to shades of brown, pink, or gray. To prevent this, pork should be stored properly in the refrigerator and used within a reasonable time.