Vicki, how exciting that you might get a chance to travel to a colder climate over the holidays.
I don't own a Soia and Kyo coat at the moment but based on try-ons I would say they fit quite slim as a rule and work well for a smaller frame (at least in the styles I'm most familiar with) and some people would need to size up for shoulders or hips or bust. But not everyone. It also depends how much (and what) you intend to layer under the coat. If you are between sizes on their charts, I would tend to go with the larger size. If you are not on the edge of their sizing, then probably your normal size.
In terms of colour, I'd go with one you love if you can find it. Black is not just classic -- it is also ubiquitous, which for those of us who live in cold climates can become a dreary sight both on the street and on our own bodies, but which those who only wear one of these jackets on a trip might prefer.
Also, consider how heavy a coat you need. There are different weights of down and this makes a huge difference to warmth. If animal welfare is a concern, check to see that the coat is made from responsibly sourced down. You can also find coats that are not down-filled that work, as well -- check out Save the Duck if this is a concern for you. Note that synthetic fillers also come with environmental costs, so there are no easy answers here.
(I do still wear down myself but understand and respect the concerns).