The darkness of knees and elbows comes from dead skin that happens to show up more on people of color. The friction of leaning on the elbows and kneeling on the knees contributes to the darkening of these particular body parts. You can always try products that lighten the skin by visiting a doctor or buying over the counter ointments. Below is a simple recipe that will help out with dark knees and elbows. Give it a few weeks to see any changes.
1. After cleaning knees and elbows rub with lemon to bleach and remove dead skin. Cucumber slices are known to do the same thing or you can combine sugar with lime juice for the same purpose.
2. Afterwards you can moisturize with coconut, olive, or some type of oil.