Meat is generally classified as light and dark. Red meat is part of the dark classification. It’s just that beef has a higher concentration of myoglobin than chicken legs so it appears redder. Pork is in-between which is why it appears pink.

And while cows don’t move much, just standing requires them to use a lot of muscles. Which is why you’ll hear about veal farms doing horrible things like chaining calves to the ground so they don’t develop their muscles. Which in turn causes less myoglobin which makes their meat whiter.