Is it "a sombre look" or "sombre looks" when there are many people having (a) sombre facial expression(s)? [duplicate]

The answer stated above as being the 'grammatical' choice sadly gives the impression that the alternative is wrong. It is not wrong (where no real scope for misinterpretation would ensue) to address the (common) individual within a group: 'Hold the protractor down with one hand.'

In this particular case, while 'Remind your wives' is certainly not wrong, 'Remind your wife' will almost certainly not add any confusion and would only be labelled incorrect by prescriptivists above a certain level. I can think of no 'grammatical rule' proscribing and am familiar with the acceptable usage of employing correctly formulated singular constructions when addressing more than one in a group (especially a largish or large group).

Dorgeloh and Wanner, in Syntactic Variation and Genre, state that

In [a] common strategy, speakers may use singular constructions [to simulate a 1-to-1 situation] despite a manifestly plural audience. Well-known instances of this are address forms such as Dear Reader in novels, editorials and so on. The pragmatic effect is quite clear: [t]he use of singular forms can be used to simulate closeness or intimacy, states which are usually absent from mass-medial communication. While 1-to-1 simulation is arguably easier where there is no physical co-presence of the addressees (for example, in the written medium or in TV or radio broadcasts), it may also occur in face-to-face situations, if the audience is sufficiently large[: t]hus sermons or speeches are situations in which ... 1-to-1 simulation may be used by the speaker.

The device of at times addressing the individual within the crowd was used in Hebrew by Moses (and his Boss), in Deuteronomy, as T A Lenchak, and doubtless other commentators, point out.