Building brick moulding in cold weather
Because the lengths of brick mold are usually fairly short, less than 7 feet usually, expansion or contraction of a wood product is negligible. We often install exterior trim in cold temps here in Maine and really never see a problem. A couple of tips that will help insure tight joints year round are to put some waterproof wood glue, like titebond III on all the joint faces and be sure to put a finish nail through the joint. Use a good quality siliconized, paintable acrylic caulk. This product will set up fine in cold weather as long as you are not sub-zero. Temp specs are usually on the product, but don't be afraid to push it a bit, won't really matter. Also there is now low temp exterior paints available.  Good luck