Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if blue or grey lips, tongue, face, or widespread skin colour appears suddenly, particularly with:
If someone is choking and cannot breathe or cough effectively, begin appropriate choking first aid while emergency help is called.
A baby with a blue or grey tongue, lips, face, or body needs immediate assessment.
Blue hands and feet may occur briefly in newborns, but colour affecting the lips or tongue should not be assumed to be normal.
And for anyone, someone who:
Seek same-day medical advice if a new colour persists after warming, even without severe breathlessness, or if a home oxygen reading is unexpectedly low or steadily falling.
And for anyone, someone who:
Book a routine appointment if episodes recur, affect daily activities, or happen without an obvious cold trigger.
Mention new medicines, chemical exposure, finger pain, sores, swelling, or changes affecting only one side.
And for anyone, someone who:
The article also says
Do not drive yourself if you are severely breathless, confused, or faint.
Every sign above is quoted from the sofidex article on Blue or grey skin or lips. If you are not sure, err upward — it is always reasonable to be checked.