Socks that match your trousers
Short answer: The sock should match the trousers rather than the shoes. That gives an unbroken line from knee to foot and makes the leg look longer.
The rule that works
Choose a sock in the same tone as the trousers. Then there is no contrast cutting the leg in half, and the whole line reads as one piece. It is the simplest sock rule there is.
The exceptions
With sportswear and very casual clothes it matters less, and the sock can be a deliberate accent. For anything that should look coherent, the rule holds. See bamboo socks for men.
What to keep in the drawer
Two or three colours cover most situations. Black for dark trousers, dark grey for grey and navy for blue. Then you always match without thinking. See all bamboo socks for men and bamboo socks for women.
Frequently asked questions
What about white socks
They work for sport and with trainers and short trousers. With dark trousers they cut the leg in half.
Should the sock match the belt
No. The sock matches the trousers, the belt matches the shoes.
What if the trousers are patterned
Choose the darkest tone in the pattern. That makes the transition calmest.