But don't most people have a funeral outfit?
I doubt that, based on the outfits I have seen, and especially not in all black. And you are really talking about two or more funeral outfits too - you couldn't wear your winter funeral outfit to a summer funeral..
Even though funerals here (US) and in Ireland tend to take place within a week of the death, and you might expect people would realise that once a funeral is announced there will be little time for shopping so it would be wise to have something wearable, most women in the congregations I have been in for funerals very clearly cobbled something dark or black together with time pressing upon them.
I had to dash out with my teenage DDs to buy clothes for them when exFIL died. It was the end of summer, right in between retail seasons, and there was literally nothing black in the shops. We ended up in second hand shops and did ok but it was a close run thing. If we have to go to another funeral (one grandmother not in great shape) we will be faced with a similar mad dash, because teenagers grow and change shape, and adults can gain and lose weight too. I think most people have some items that are blackish or dark or generally sombre that they take out of the wardrobe, dust off, and say, 'It will have to do'.
I have been to many Catholic funerals, some Unitarian and some Lutheran in the past ten years. The attire has been formal, with lots of black visible, but I have never been in any congregation where even 5% of the people wore all black, even when the death was tragic and the community was overwhelmed by grief (three suicides and several untimely deaths).
I normally wear a lot of black and dark grey but not necessarily in a smart style. For a funeral I personally have a pair of smart black trousers for cold weather and a pair of black cigarette pants for warm weather, a top with a black background with small dark purple and dark green dots for warm weather and a cardigan to go over that if it's cold, a pair of smart black shoes, pair of smart black sandals, and a pair of black boots. Also a black wool overcoat. None of the blacks go together really. Overall I would describe my look as 'funeral frump'.
For men, black shoes and a black tie with a suit makes it easier. I have seen jeans plus black shirt however. Absolutely nobody would ever get sniffy about what someone else wore to a funeral though.
I think the only thing that would cause comment would be turning up in all black to the funeral of someone who was not a close relative (not immediate family). I think that would be considered presumptuous or attention seeking.