I would wear a simple stretchy dress (a jersey fabric) paired with a cardigan (I live in the Netherlands, so I feel your pain! Or rather, your cold...). Something printed, maybe. My go-to dress for informal parties is a simple black jersey dress (I also have one in a dark plum). I dress it up for weddings in a pinch (pair it with heels, a jacket, done-up hair, nice jewelry) or down for an informal party (sandals or flats, cardigan, I'd wear my hair down, simple silver jewelry). If it's too cold for bare legs, go for a simple dress in a darker color and wear it with leggings (quite informal, of course) or pantyhose.
You could also opt for elegant pants and a cute, colorful top. Add a cardigan, again. I have cardigans in black, grey, and three different colors - my go-to items!
Have fun!