Each case is different, I'm sure but....
My dd gets blue lips when she is cold. It's only when she's cold. When she is running around butt naked (because she refuses to put clothes on) and I am cold with sweat pants/shirt on, sometimes her lips turn blue. Around that time, she agrees to put clothes on. *shrugs* It doesn't concern me at all. Her veins are very present throughout her body, her lips are blue and her skin is cold. This has happened periodically for months....when she's naked and cold. (FTR, sometimes I force clothes if it's really cold in the house. But I usually let her happily run around naked because clothes aren't worth the fight).