Flashes are nice! You can buy them or make them yourself.

There also tartan flashes to match your kilt (sometimes you get them with your kilt, sometimes you must order them seperatly)

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.