Depends on the type and depth of skin color.... Do direct online consultation  by sending photos, otherwise meet me at my clinic, for accurate diagnosis and treatment

self medication is reckless and unsafe. it is not to be taken life long. it alone won't be enough. The darkening of skin can be multiple causes like tanning, vitamin B12 deficiency, addisonian pigmentation, acanthosis nigricans, lichen planus pigmentosus, rhiel's melasnosis or several other causes. A proper assessment and examination will help ascertain the proper cause. For loss of glow & dullness - there are antioxidant therapies like glutathione , lasers for rejuvenation, PRP therpy for facial rejuvenation , peels, etc. You should send me images through private online consult for a better assessment of the problem and approaching it accordingly. Hope this is helpful.