Hello, Welcome to the magical world of health care, I went through your query, and the colour is the collection of blood as a sign of injury. you need to look for 1 any instability or odd feeling of falling while walking 2 persistent pain. 3 Swelling of the knee joint. if any of these is present an MRI of the knee will be needed. I hope my advice would have been useful, in decision making regarding your treatment, still if you have any clarifications or doubts feel free to contact back. Thanks.