The skin over the elbows and knees are particularly elastic and resilient. You will notice that the pigmentation is more apparent when your joints are extended and the skin overlying the joints is contracted and redundant. The pigmentation is a reflection of the type of skin around the joints and the fact that there is constant friction between these thickened areas and the environment.
You are correct that the treatments you mentioned are temporary. In other words, there is no cure to change this specific skin type because the skin in these areas serve a specialized use. Permanent depigmentation can be acheived but may be contraindicated based on your native skin color.
I would recommend a chemical peel or a more regimented exfoliation using prescription strength lotions. The most important aspect of this answer is that the condition cannot be cured but only treated with vigilant and longterm treatment.