The white shirt issue must have been one in many congregations. I can remember on one occasion the CO was told to change into a white shirt. That is what probably prompted this in the WT.
*** w89 11/1 p. 30 Do You See Only the Outward Appearance? ***We can illustrate this in the matter of dress. The Bible requirement—God’s requirement—is that a Christian’s clothing must be neat and clean, well-arranged and not betraying a lack of “modesty and soundness of mind.” (1 Timothy 2:9; 3:2) Obviously, then, the elders in one congregation went “beyond the things that are written” a few years ago by requiring every public speaker in their congregation to wear a white shirt, even though pastel colors were generally acceptable in that country. Guest speakers who showed up with a colored shirt were asked to change into one of several white shirts kept in the Kingdom Hall for just such emergencies. How careful we have to be in order not to impose our personal taste on others! And how appropriate Paul’s counsel: “Let your reasonableness become known to all men”!—Philippians 4:5.
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This was evidently only applied in congregations. Ten years ago at the last DC I went to, one of the speakers cancelled out at the last minute due to illness. The DO selected an elder but demanded he go home and change into a white shirt. White shirts okay at the congregation level but not at assemblies or conventions. I wonder what scripture that is.