hi..thanks for your query..
A lot of people have dark lips genetically predetermined,although parents may/may not have it. A frequent cause of non genetic lip darkening is the use of lipsticks /coloured balms/gloss , as these tend to darken the lips over a period of time.So try and not use it too often..If you have to use it,then let it be allegen free and replace them every 2 months.I would advise you to start using medicated lip balms with SPF and to reapply it several times in a day..that helps in reducing further darkening.Also use a cream containing kojic acid
twice a day for lightening of lip colour.U can also opt for mild chemical peels
once in 15-30 days to exfoliate better and lighten faster.