Is it appropriate for guests to dress in all white to a wedding? Nope.
Here is the thing; I don’t think it is appropriate to dress in all white as a guest of a wedding - whether or not the couple is wearing white. With the exception of “all white” wedding dress codes but those seem to have mostly passed now.
My background is in theatre, so I will explain this in relationship to theatre. If you are watching a play with a number of characters and one of them is wearing all white, you will be drawn to look at them. This is a technique that costume designers often use to instruct the audience of where to look. Similarly, if you paint a room in your home all white, it will reflect the light and make the room brighter than any other. The reason for both of these examples is that the colour white reflects light, so it quite literally is the brightest colour in the room.
If you wear all white at a wedding, you are unintentionally taking focus away from the couple and putting it on yourself. Of course, there are some couples who do not care about this and would be happy for you to wear white, but that is likely something you won’t know and it is not appropriate to ask.
Now whenever I discuss this, folks ask me if this is why brides often wear white. The answer is no. Wearing a white wedding dress in the modern age is simply a trend that never went away. Queen Victoria wore a white dress for her wedding in 1840 and other’s loved the look and grasped onto it.
If you find yourself conflicted with the political and social implications of wearing white, know that no matter your decision, wearing white is simply an act of following a fashion trend.
Interested in the history of other wedding trends? Let me know, I would be happy to explore more in the journal.