the "wear beige" thing is really a snappy way to say, "don't make things more difficult," I think.
And "you are not the most important person here" and even "you aren't the decision maker."
I think that in the days when mothers were greatly more involved in planning weddings, it would be easy to end up in a situation in which two women who were used to ordering and organizing their own households might instinctively end up trying to both do the "steering."
I think those two underlying thoughts (don't make it difficult; you aren't in charge) are still good things to be reminded of.
Agree. Also it was from a time with the bride's family normally footed the bill for the wedding and reception and they were not considered a host of the event, just an "honored guest".