what does a black and white dress code really mean?

[insancerre]

I have been invited to a works do and the invite says dress code is black and white.
Do I have to wear a long black formal dress?
Or can I wear a patterned dress as long as it is black and white?

Black and white pattern should be fine!

I think they just mean 'dress in whatever as long as it's black and white' I'd assume they'd be expecting about a 'cocktail' level of dress. in any case they don't say 'black tie' so full length and massively formal wouldn't be necessary.