Ok, I was having a hard time figuring out what type of salt to use. Some people say flour, some say regular granulated, some say fine. I've tried floured and regular granulated (granulated was damn near impossible for me to keep suspended).
This is by no means scientific like but what I did was take equal amounts of vegetable oil (I know this a bit thinner than hot plastic but it's just a test) and put in some glasses then measured out 1/4 tsp of 4 different salts...course, regular, popcorn (fine) and floured. I put the salt in, gave it a stir then timed how long it took to settle out. The course and regular were pretty much immediate. The fine took between 30 and 45 seconds to completely settle out. The flour took about 1.5 minutes to settle out (there was still some suspended at this point but the majority had settled). The fine was much clearer with suspended salt than the floured. The floured was very cloudy and opaque. The floured also formed some small lumps when I put put it in that were not easy to completely break up. I could easily see them in the oil but never would have in colored plastic. Although the floured stayed suspended much longer it appears to me, based on this test, that the fine salt would be the better choice. You have enough time to pour some molds between stirs and will definitely require less color than the same color attempted with the salt flour.
I just wanted to share my findings in case anyone else was wondering what to use.
Greg