I will have to agree that Pantherella's are some of the best socks out there. That is all I wear, from wearing a suit to wearing a pair of jeans, they are always on my feet. They seem to hold their color extremely well, and much of that is because the socks are yarn dyed.
Nordstroms. Yes, Nordstroms. They have some with blue tops (a thin strip of color) to signify cotton blends and others with maroon tops to signify wool blends (I may have that backwards).
Actually the strip of color on the Nordstrom socks is not to differentiate material, rather, it is to differentiate color. Navy socks (both cotton and wool) have a light blue strip on the top, black socks have a maroon strip, and brown have an off white strip (I think).