I am learning a lot about coleus here, but one question I have not seen answered, and perhaps I have just missed it - is about how cold a coleus can get and still survive. I just bought some beautiful plants at Costco (yeah, I know... not the best way to go!) and one is already dead, the next is not doing well. I have left them outside, but after reading the threads here, I am thinking that even though we are getting up into the 80's some days, and stay in the 50's other days, perhaps the nights dropping into the 40's are too cold yet? Should I bring them in for a while to nurse back to health? They are in light plastic "pots" about 6" in diameter at the most.

Good Morning Washingtonian! Coleus are great additions to any garden. As you already know, coleus are a tropical plant and really are drama queens when exposed to what they believe is inclement weather. There are a lot of factors in regards to how much cold they can take - have they been properly hardened off before these cold nights have set in, how long did it stay cold, humidity (dew point), maturity of the plant, etc. I have had similar issues with coleus in the past - they don't seem to want to come out of a cold injury fast, if at all.
Back to your question - how much cold? They really don't like even low 40's at night. (I tell mine that it doesn't freeze until 32 (plus or minus) so you'll be fine...but sometimes they disagree)
Good luck and enjoy your coleus!

SOmetimes pulling a clear plastic bag over them would be like a greenhouse.

I run into this problem here in NJ in the Spring as I begin to move my Coleus into my Greenhouse. This year was a cold Spring with many nights dipping into the 40's which will damage or kill young Coleus.
I use a Ceramic Disc Furnace to boost the indoor temperature overnight to keep the tender young Coleus warm.