First you can use cucumber- cucumber is the most recommended method to use because it's got some bleaching agent in it and will thus give the knees its original skin colour. Cucumber removes the dark skin cells and keeps the skin moisturised. What you have to do is slice cucumber into pieces the take one piece and rub it on the dark knees for around 30 minutes then let it dry up for around five minutes and then rinse it with water.
You can also use potato, it contains enzyme catecholase that lights up the skin and gives it a tone colour. Just like cucumber, you can the potato into two then squeeze out the juice and then apply on the dark areas for some time and finally rinse it with plenty of water.
Finally, you can also use honey plus lemon juice or baking powder. All you need to do is make a mixture of the two then apply the mixture on the knees the that same period mentioned above then wipe away with water.
What you have to keep it mind in the process above, its not a one day process, you have to keep on repeating the procedure for at least one month to get the better results.