Yellow, red, black
Physically, lions are a tawny golden color, and are the only cats whose sexes can be told apart at a distance. Males bear thick, shaggy manes of either blond, black , red, or brown, while lionesses are barren around the neck. Both sexes have a tail tuft, always black in color, though cubs are born without this tuft. Lion eyes are most commonly amber, but brown-eyed, green-eyed, and blue-eyed lions have been observed. A male lion is roughly twice the weight of a female, being around 420 pounds, while lionesses only weigh about 290. Rarely, a pair of lions will give birth to a white lion. The age of a lion can be told by examining how dark its nose is.
In the Real World
Lions are found in southern Africa, Asia, and India, though they are considered critically endangered in India. In Africa, they are considered "vulnerable". Lions used to be plentiful in western Asia and the Middle East, but they have long since decreased in these regions. Lions usually live around 10 to 15 years in the wild and have been known to live for up to 20 years in captivity. However, it's rare for a lion to live into old age. Many die from either starvation or injury, as a consequence of other violent lions.
The average pride is comprised of ten lionesses and is protected by anywhere from one to four males. The lionesses are usually related; either mother and daughter, sisters, aunt and niece, or cousins. Outside female lionesses are rarely accepted into an existing pride and seldom leave their natural prides to begin with. However, a takeover can prompt them to leave, as well as overcrowding in the pride. When a lioness with adult children leaves the pride, her daughters typically go with her.
While males are able to hunt, lionesses are the main source of the pride's food. Lionesses possess great speed and the ability to run long distances without getting overheated. A lioness can run about 40 miles per hour, and their cooperative nature - highly unusual among the cat family - allows them to take down large game such as buffalo or wildebeest. Larger prides have been known to fell elephants and giraffes. Lions mostly scavenge and steal .
A group of lions is called a pride. There is no hierarchy or true "alpha female" among the females of a pride, as females mate freely. However, if there is more than one male in the pride, there will be a dominant male among them, and he will sire most of the pride's cubs. The strongest, healthiest males have black manes, while weaker, unhealthy males have short blond manes. Lionesses are attracted to darker-maned males. A male's mane color is not static and changes with the state of his health.  Among other males, the black mane is seen as threatening, and a male will hesitate to attack a lion bearing one, even an intruder.
Male lions function as a barrier between the pride and invaders, marking territory with scent glands and urine, and sire the lionesses' cubs. Whether or not the lion is a tolerant sire or a hostile one varies from male to male. Though primarily a male's job, females will not hesitate to defend their cubs from marauding males.
Young male lions reach sexual maturity between the ages of two and three years, after which the dominant male of the pride will begin to see them as a threat and drive them off through violence. Along with assuring their power, the male also prevents incest within the pride. Males who are ousted together form coalitions and, like the lionesses of their natal pride, are usually related. However, they may be unrelated. Coalitions are typically pairs, but trios, quartets, and even sextets have been observed.
Male lions are not as fast or efficient as a pride or even a lone lioness, as their manes cause them to overheat faster than a female. As a result, many males die of starvation before they ever claim a pride. If they do manage to beat out an existing male, however, they will run off any existing males, kill the cubs that are too young to flee, then mate with the lionesses, completely erasing the old leader's bloodline. A male's tenure as protector and ruler of the pride generally lasts two to three years.
Lions are apex predators, and the closest thing they have to competitors are hyenas. The real danger to them are their own species and humans. Rival prides have been known to get into deadly clashes.
In the Universe
The lions in the film series look and behave markedly different from actual lions. The Pridelanders are the main group of lions featured throughout the franchise, though other prides such as Malka's pride and the Outsiders have been lightly touched upon.
The Pridelanders operate in a monarchy system, where the firstborn child of the current ruler serves as the crown prince or princess. This child is anointed, then presented before the animals of the kingdom in a ceremony that occurs at dawn. Sometime after, the ruler-to-be is assigned a lion or lioness whom they will marry upon reaching adulthood, though this tradition appears to have been broken with Kiara.
Though hyenas are their sworn enemies and portrayed as such in most material, lions have been known to conspire with them. Scar maintained a friendship with hyenas from young adulthood to late adulthood, and both Kion and Kopa befriended a hyena, Jasiri and Asante respectively.
True to their species, lions have rival prides, as seen with the Outsiders and Pridelanders. Power can be stolen through usurpation, as seen with Scar and Simba. The lion Tanglemane was run out by a pair of rogues and lost his crown in the process.
- In the first movie, hyenas take on both lions and lionesses. In reality, hyenas only mob lionesses if there is a three- or four-to-one advantage. However, male lions are avoided at all costs. Even if the hyenas outnumber a male twenty-to-one, the hyenas will give him a wide berth.
- In The Lion King universe, only the male lions have whiskers, while the lionesses have none.
The Lion King Wiki has a collection of images and media related to Lions.