Sorry to hear that you injured your knee! The color changes that you are describing are not in and of themselves alarming. A bruise
is essentially leaking of blood under the skin. As the bruise heals, your body digests the blood away, and the chemical changes involved in the digestion of the blood leads to these color changes, first from purple or blue then to a greenish or yellowish or brownish color that fades away.
I would still suggest consulting your primary care doctor
, however. Although the bruise itself is probably not alarming, it is important to make sure that you didn't cause any major damage to the tendons and other structures in the knee itself. Your doctor should be able to rule out major damage with a simple physical examination, although sometimes other tests, such as x-rays, are also needed.
At the same time, your doctor will be able to give you advice on what you can do to speed healing and prevent further damage. This might include recommending avoiding vigorous physical activity for a certain period of time, taking anti-inflammatory medications, and the like.
Make an appointment with your doctor at your earliest convenience.