ArtResin's ideal working temperature is between 72-77F or 23-25C. Resin colder than this is thick, clumpy, hard to work with and looks milky because it's full of thousands of microbubbles (which you'll never be able to torch out.)
Carefully warming up the unopened resin/hardener bottles in a warm water bath for about 10 minutes can greatly mitigate this issue. Be aware that warming your resin in a waterbath cuts down your working time by about 10-15 minutes, so time yourself accordingly.
Dry your bottles thoroughly after removing them from the waterbath - even a single drop of water into the mixture can cause it to turn cloudy and prevent it from curing properly.