You'll notice a color change in juniper and boxwood in response to the cold. *The green color of Juniper, boxwood and pine may tend towards a golden green or rust in some species.
Evergreens don't become entirely dormant so long as there is light and moisture available for them to grow. *Their metabolism certainly slows down in response to colder temperatures.
[*]Alberta spruce is fine on your growing benches. It's among the most cold tolerant plants you could grow as bonsai.
[*]Here is a nice chart of boxwood zone hardiness. *A lot depends on your growing zone and the species you have.
http://www.westernmainenurseries.com/basket/woody.asp?action=list&currentpage=3
[*]Look at this chart for addional species, including common garden juniper - Juniperus chinensis -30 to -20F. *
It will be less for bonsai grown in the ground, but if you get the trees off their benches and onto or even into the ground, by burying the pots slightly in the earth, or storing them under the benches under mulch, out of the full force of the wind I don't think you will have problems. *Some growers protect their trees by putting them under the benches and covering with white polyethylene sheeting
http://www.westernmainenurseries.com/basket/woody.asp?action=list&currentpage=3
P.S. I need to admit here that I have no direct experience with the kind of weather you're dealing with. Our winter here in CA is balmy by comparison.