I think this is probably down to unfortunate phrasing - at first blush it looked like you're doing the [citation needed] thing on someone who was just adding an interesting anecdote to the brown/red band situation. However I assume you just wanted to say that this info would be an interesting addition to that page, and if OP had any idea whether there was any more official info on this that would be cool, since the anecdote alone likely won't satisfy wikipedia editors.
Yes, at the time I wrote the original comment, in the Wikipedia page linked there was a comment in the lead section of the article: "The Royal Mail no longer uses red rubber bands.{{Citation needed|date=June 2020}}". And given that the comment I was replying to seemed to be from someone local I thought they might be in a position to have or find a reference and update the Wikipedia article with that, since other comments here seemed to agree that the rubber bands are not currently red.
Observing non-red rubber bands doesn't make a person more likely to have a link to a Wikipedia-accepted source article about it, right? So the person suggesting a Wikipedia edit may just as well Google it themselves.
Yes, you are right I did not try to search for it myself before writing the comment. However I did just try and it is harder than you might think to search for something like this. I suspect if I had a better understanding of British English I might more readily stumble onto an appropriate phrasing that turns up a source.