Why does the color of lipstick sometimes change on the lips?

sorry,i dont know because i dont use lipstick..may be because of thick coat and thin coat like nailpollish!!!

The skin that covers the lips need to breathe, it needs oxygen supply to properly circulate the blood in the lips. Too much lipstick application can cause lip discoloration because lipstick blocks oxygen, plus the temperature increases in the skin covering of the lips leaving it dry and flaky... That is why I never use lipstick, I just use petrolium jelly IF my lips need to be moisturized...

yes this happens due to the lack of water or excess heat. you have noted even the smokers lips too get dark. that happens due to excess intake of harmful co2 gas that generates heat. so better have a fresh air at 5 am. and drink a plenty of water in the day to day life