Some people have darker skin on the elbows and knees. Sometimes it can be due to vitamin B12 deficiency (common in vegans), it can also be due to dry skin or a condition called acanthosis nigricans where body folds become darker. Get your hemoglobin levels and serum b12 levels tested to rule out b12 deficiency, get a fasting and post prandial insulin test done if you are over weight, to rule out acanthosis nigricans, use moisturisers if your skin is dry. Mild keratolytic creams will help in improving the condition if there is no underlying problem in your blood test reports. Home remedies like lemon juice can be tried but how effective it is, is open to debate. Warm regards. Dr. Rajdeep Mysore. Chirag Hospital Bangalore