It may have just been coincidence, but I just got a blue and purple Sign of Logic killing the Toxic spiders in Crimson Badlands (haven’t even finished the quest yet). Not sure if there’s anything to it aside from dumb luck, but those in need of this particular sign might as well mess about and see if it’s a thing
Yarp, most of the signs drop from related mobs (the higher level the better). This holds true for all of them except the stupid fusion ones I need (since they’re not a single element…maybe I need to farm Drusera? :P).