Welcome to HCM,
It seems that you got injury
on knee joint might be having some injury to your cartilage
There might be having some hematoma
giving rise to swelling and color change.
Consult orthopedic surgeon and get examined.
Give complete rest to the part.
Do ice pack application 3-4 times a day for 10 minutes each time.
Take anti-inflammatory medicines for pain and swelling.
Ok anfd bye.