I understand that it's an evening wedding and a private event where Catherine didn't expect to be photographed, though I do agree that this is a little strange for a wedding. Even though we can wear any colour we want to a wedding, I do think Catherine should've worn something a bit more cheerful (like red or something), and agree with Blue_ that it looks like she's attending a funeral. I love her scarf however, and to be fair it does add a touch of colour to the outfit.
I like keeping with traditions so I'm probably biased, but I think we should wear brighter colours to weddings as it is a celebration, whereas we should wear more darker colours to funerals as it is a solemn event. Having said that, nowadays I think funerals in the UK are viewed more as a celebration of the deceased's life rather than a somber occasion (though if the deceased person died in a more tragic circumstance or at a very young age I guess it wouldn't really be referred to as such), or at least this is what we called my great-aunt and great-uncle's funerals when they died. People wore bold shades of blue and pink to their funerals (though I opted for a more traditional outfit) as it was more of a celebration of their life and remembering the happy times we had with them.
(Apologies as I've realised I've gone rather off-topic.)