Guest
One outfit that fits into a palette you did not choose, in a room you have not seen, alongside people you cannot coordinate with.
What governs it. You are in other people's photographs and you have no control over the composition. The only thing you can control is not being the thing that draws the eye.
Where it goes wrong. Landing near the wedding party's colour without matching it. A guest in almost-the-wedding-party colour is confusing in every group photograph, and it is a more common error than wearing anything actually prohibited.
Direction. If the couple published a palette, sit clearly outside it rather than near it. If they did not, a mid-depth colour with moderate chroma is almost always right — visible, not competing, and it photographs cleanly.
In photographs. This role appears in a large share of the photographs. Separate pieces by lightness rather than by hue — lightness is what survives a camera.