If someone feels self-conscious about grey hair, mocking or ranking them lower is a failure of respect. People at every stage of life deserve dignity; the effects of aging should not become material for jokes or humiliation.
The specific prohibition here concerns plucking grey hair. It would be unsound to derive every separate rule about grooming or hair color from this one short statement.
The pronoun هُ in فَإِنَّهُ refers back to الشَّيْبَ. نُورُ الْمُسْلِمِ is an iḍāfah construction: “the Muslim’s light.”