Black.
I actually find that having all black socks makes it harder to match up because there are different breeds of black sock or even worse, the same breed but different age.
I cannot stand wearing one M&S and one Sainsbury sock, or two M&S socks with different amounts of wear.
The ones with coloured heels and toes would be ideal but I wear shoes which would mean the colour shows.
Maybe I should go out with £50 and buy many pairs of identical socks and start afresh.