My beer is 40°f. It is in a room at 70°f and 50% humidity.
Will it stay colder sealed, or would evaporation cause the warming be about the same?
Congratulations! 
Evaporation would tend to keep the beer cooler, compared to without evaporation.
I’d expect this to be a very small effect, though, and that air currents being able to enter the bottle opening would let the beer warm up a little quicker than with the lid still on. This would also be a small effect, but probably not as small as the effect of evaporation of the beer.
As your attorney, I advise you to drink the beer.
OK.
What kind of beer?
Glass bottle, plastic bottle, metal can or pub glass?
I had a beer. I may have another beer.
I tried to test but the open ones don’t last long enough to evaporate.
Moving from IMHO to MPSIMS.
I have many, many beers. I am attempting to rectify the situation.
I will update as conditions warrant.
(zzzzzzz)
You had me at “beer”
-D/a
I’ll see your beer, and raise you a 12-pack.
Why do they put a use by date on beer?
I would rather have a bottle in front of me than a frontal lobotomy.
You’ve answered your own question. If it lasts long enough to evaporate, you’re doing something very wrong!
I have had 5 beers and am feeling fine, thank you very much.