The trench coat is usually on any list of "10 essentials that chic Parisians wear", or articles and books on a similar theme, however it tends to be accompanied by words emphasising the importance of it being in a neutral colour such as black, navy or beige. Would a Frenchwoman wear Catherine's gorgeous trench? I'm guessing that they would if they really loved it and knew they would look as good as she does wearing it.
In the photo below, I'm wearing a trench coat that I bought last autumn. It's not in a traditional neutral colour, however the copper isn't a trendy colour that will date, so hopefully it will last me several years.
I'm wearing the trench coat over dark brown jeans and shoes, plus my tan bag doesn't provide a lot of contrast. My colouring is low colour contrast, so the combination is good, however I am also high value contrast, because of my pale skin and dark hair and eyes, which is why I wore this particular scarf. What you won't be able to see is the pair of green earrings I was wearing, so I will point out that the green earrings went beautifully with the green in my scarf. It's these little details that makes choosing an outfit more fun.
Finally, this is me wearing what I like, but what about you? Do you worry about wearing clothes that aren't age appropriate? On the other hand, do you want to ignore what so-called fashion experts decide are the styles that chic women should wear? By the way, I include myself in the latter category!