About kilt socks (they're called hoses...don't ask me why)...the pure white hoses are for pipers, the off white/cream/arran hoses are worn when dressed up. All other colours (oatmeal/stone, lovat and navy blue, lovat and bottle green, charcoal etc.) are for more casual occasions.
Actually, coloured hose may be worn for formal occasions as well, not to mentioned diced and the "Argyll" or tartan hose, which usually match the colours of the kilt.

Cheers,

Todd