Growing Pains: This condition is common in children around the age of 5, characterized by recurrent leg pain, typically occurring at night. The pain often resolves by morning and can be exacerbated by physical activity. The fact that the pain is worse at night and the child experiences leg discoloration on some days, possibly due to changes in temperature or activity, could align with growing pains, although the discoloration aspect is less typical.

Juvenile Idiopathic Arthritis (JIA): This condition can cause joint pain and swelling, which might be worse at night. While it more commonly affects the hands and knees, it can also affect the feet. The leg discoloration could be due to inflammation.
Trauma or Overuse Injury: Given the child's age, it's possible that the pain is due to a minor injury or overuse from playing, which could be exacerbated by activity and relieved by rest. The discoloration might be related to bruising or changes in circulation.
Peripheral Arterial Disease (PAD) or Vasculitis: Though less common in children, conditions affecting blood flow could cause pain and discoloration, especially if the symptoms are activity-dependent.

Osteomyelitis or Septic Arthritis: Infections within the bone or joint can cause severe pain, especially at night, and could lead to systemic symptoms like fever. These conditions are medical emergencies requiring prompt treatment.
Neuroblastoma or Other Tumors: Though rare, tumors can cause night pain due to the body's position or increased pressure on the tumor at night. Leg discoloration could be due to compression of blood vessels or nerves.
Sickle Cell Disease: If the child has sickle cell disease, episodes of pain (sickle cell crises) can occur, including in the feet, and might be accompanied by changes in skin color due to vaso-occlusive crises.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. It could potentially cause joint pain and skin changes.
Complex Regional Pain Syndrome (CRPS): A condition characterized by chronic pain, inflammation, and hypersensitivity in one of the limbs after an injury or trauma. It's rare in children but could explain the pain and discoloration.
Fabry Disease: A genetic disorder that can cause pain in the hands and feet, among other symptoms, due to the buildup of a particular type of fat in the body's cells. It's very rare and typically presents with other systemic symptoms.