Why is it that in the "good old days" (I've been cooking/ buying/butchering for 6 decades) freshly ground meat was red until prolonged oxygen exposure turned it brown (store-bought, packaged meat was a bit greyer...darker red on the outside and nice pink/ red on the inside). Today I notice the opposite, red on the outside and grey on the inside. Why is this happening?
The color of meat comes from the pigment myoglobin. Freshly cut meat shows a purple-red color. When exposed to air, myoglobin forms oxymioglobin, providing an attractive cherry-red shade. However, long exposure to air and store lighting lead to the formation of metmyoglobin, which is brownish-red. The inside part of the meat can show a more greyish color due to the lack of contact with oxygen.
The industry uses some techniques to control / cover the oxidation of myoglobin up to metmyoglobin and keep a nice red shade for a longer time.
Modified atmosphere packaging (MAP): to avoid complete oxidation of the meat and final brownish/greyish color (that does not look appealing anymore); some gases like carbon dioxide and nitrogen are used to decrease the content of oxygen and to slow down this process. Therefore, the outside doesn't turn grey that fast. For more information please refer to: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4093050/
Color additives: in Europe, minced meat is a product category that is subject to contain color additives according to the Regulation (EC) No 1333/2008. E.g. the category "burger meat" can contain carmine or carminic acid (red color extracted from crushed bugs and stabilized with aluminium salts...), whereas other categories within the group may not be allowed to contain such color additives but natural colors or vegetable extracts. Nowadays many meat producers are switching to red beet or radish extracts to enhance the red shade of fresh minced meat in fresh sausages, meatballs, etc. while keeping "clean label". In the US there is a list of commonly used meat additives (https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/food-labeling/additives-in-meat-and-poultry-products/additives-in-meat-and-poultry-products)
Note: nitrites and nitrates are food preservatives not used in fresh meat but in cured and/or processed meat.