You can use Lemon and Potash to lighten your dark Elbows, Knees and Knuckles because, first of all, what matters is what the ingredients contains

Lemon(as a Skin brighter) you can check out how to get rid of Dark Elbows, Knees and Knuckles Fast naturally at home nella's comfort on YouTube
https://www.youtube.com/watch?v=2_c7ddrSoVg

It is rich in vitamin C and citric acid. vitamin C is a great antioxidant for neutralizing free radicals and boosting collagen production. This means that Vitamin C can help lighten age spots, dark spots or a fake tan gone wrong.

Lemon (as a shine eliminator)

Lemon is great for reducing excess oil

Lemon (as an Exfoliates)
It can be used as lip exfoliator

Tip: Put a little Lemon juice on your lip before bedtime and Wash it off in the morning to help remove dead skin cells and dried skin.

Warning: Do not try this if you already have severely chapped lips

Lemon juice acts as a natural exfoliates as it contains citric acid which helps to peel and remove the top layers of the dead skin cells


Potash
https://www.youtube.com/watch?v=2_c7ddrSoVg OR nella's comfort on YouTube
Potash is edible and is used for cooking.
it's used for treating toothache
it's used in administrating cough treatment
It's used for fertilizer production
Potash acts as a preservative

You can check out how to get rid of Dark Elbows, Knees and Knuckles Fast via:.
https://www.youtube.com/watch?v=2_c7ddrSoVg
or nella's comfort on YouTube