What is "Black tie optional" in Chinese I'm writing a wedding invitation in both Chinese and English.  In English, the dress code is "black tie optional", which means guests are given the option to either wear a tuxedo or a proper dark coloured suit.  What would the equivalent of this be in Chinese?

Candidates I have so far are


正裝
禮服


Neither seem to accurately capture the equivalent of "black tie optional".  Any other suggestions?  If it matters, this invite will mostly be sent to people in Hong Kong.
In the end, I went with:


  禮服或正裝