The red potions will reveal how many unatoned sins a soul still has. 6 being the highest, 1 being the lowest (or 0 with a purple potion!).
Essentially if you have a 5 or 6, you may want to void that applicant, where a 1 or 2 would be safer to approve. If I make updates to the game in the future, I'll make that more clear. Thanks for playing!