Going to a wedding can be very expensive and buying a special "only really looks right at a wedding" outfit is not for every pocket. Most people feel very smart and glamed up in an outfit that happens to be black - so may not even think about the fact that it's a colour that doesn't always display to best effect outdoors during daylight.
I'm just pleased when guests make an effort to dress for the occasion, I think it's a little disrespectful to the happy couple to make no effort at all. Of course it can be difficult to get it right. I went to my sisters wedding in a black and white floral dress, little black cardi and a black tifer and I felt perfectly at ease and looked the part in the photos even though everyone else was more formally and colourfully attired. My mother was most complementary "just right" she said with approval.
At my son's wedding, my mother turned up in a simple summer frock and cardi in a lovely magenta pattern. I was furious! She looked scruffy IMHO and I felt she should have made more of an effort - it didn't look like she had put any thought into her outfit at all. Yet it turned out that she gave a very generous cash gift to my son, so she did care, just her outfit didn't fit my ideas of what my Mum should wear. I was the one with the problem, not her.