what side effects occur to plants when a growroom is to cold
other then them dying?

a temperature decrease during the dark will change colors of the leaves

any color in particular

A cold room typically stunts growth depending on severity. In general, you won't want your temperature below 20 degrees, but it can still survive and flourish at 17 degrees. Just don't grow the stuff in the freezer

usually it turns purple depending on your strain

say if it gets to 30 degrees at night can i still grow a good crop

well i only know farenheit and i wouldnt put it nowhere below 65F at night