Fingernails look pinkish because we have red blood in our capillaries, the tiniest of our blood vessels. If we push on a nail, it forces the blood back out, up our arterioles (the narrowest of the arteries), so the nailbed turns white for a split second. Then the blood rushes back in within 2 seconds, so the nailbed looks pink again. This is called “Capillary Refill“.
However, if a person is in Shock, their blood pressure is very low, and blood hardly circulates. Consequently, it takes a lot longer for blood to get back in the nailbed; over 2 seconds, maybe a lot more. As such, delayed Capillary Refill is a reliable sign of poor circulation, in particular during Shock.