hmm - agree that the dress needs something subtle - definitely not a bright colour or a pastel, but perhaps a shrug in a dark colour - plum / dark teal etc, or even something muted and metallic like bronze or pewter.
Wearing colour with black to 'brighten it up' almost always backfires - better to go with something understated and elegant.
Actually what strikes me as being perfect is a lacy or sheer shrug to echo the detail at the hem - this is fantastic - black with a subtle metallic detail that you could pick up in shoes / bag /jewellery.
This might work too if the colour is good IRL, but comes in black if the purple's not right (though you have to buy the vest too). Am not convinced about the tie front though.
This is very chic but might be a bit structured to go with such a floaty dress - this one is worth a look too but might be too fussy.
However, I think this one could be just the thing as it would add interest without brash colour - black-and-white accessories also very cool.