I had the same problem as you with different states of wear/color over time. So the last time I bought a whole bag of matching socks, when I first took out each pair I put a matching mark on the inside surface of each sock (like 1 dot, 2 dots, 3 in a row, 3 in a triangle, different color mark, etc). So then I can easily pair them all up correctly after I do the laundry.