Purest white is kinda dependent on your display (settings and panel tech), eyes, where you are standing in DDO (as some environmental lighting with alter the colours viewed), as well as how textured the item is.
For example, because the White Dragonscale Armours are latticed with the scales as a texture, it kinda deadens the White, as well as it being matched up with a lot beige, yellow or gold coloured acents so it doesn't quite feel as pure white as it should be.
Whilst something like the Cooks Outfit is a lot more white because there's a lot less to break up the white colour.
However, both will look off if you stand in the Feywild with the blue environmental tinge. Or in Eveningstar during sunset where everything is awash with a yellow red hue.
J1NG
:: edit ::
Silly me, I missed out the recommendation list:
"White" garments:
White Dragonscale equipment, VIP Silver Dragonscale equipment, Robe of the Magi (due to more unbroken blocks of white), Unversity Bulwark stuff (if you're only after the top as white), and Cooks Coat would be your best bets.
Black cloaks:
A random gen can be a solid black colour, so sweep the AH (non ASAH) for all the cheap cloaks, put them on then dispose if not black. The other option is as
@Hobgoblin said, the Vampire Cloak, but that's a DDO store item, so may not be for you if you prefer not to spend DDO points on it.