Quoted from an extensive and in-depth guide from a different (and far superior
) site which I won't link to, since I'm not ISO a temp ban, from an article written by someone who knows far more about the mechanics of sins than any of us:
Quote
Q: Does the -35% Enemy Lightning Resistance on the 'Crescent Moon' runeword and the Lightning/Fire-flavored Rainbow Facet help the Traps and FoF/CoT/PS do more damage?
A: No for the Traps (as the game considers them minions, they do not benefit from -x% Enemy Resistance), and yes for CoT/PS/FoF. Do note that while -(3-5)% To Enemy Fire/Lightning Resistance on Rainbow Facet does not work with Traps, ATM, we have not confirmed whether the +3-5% To Lightning/Fire Skill Damage on it does (though it has been confirmed not to work in SP).
Both FB and SWeb are exempt from the above bugs/penalties, and benefit normally from Rainbow Facets (e.g.. both -% enemy resistance and +% skill damage work on them), as the game does not consider them true "Traps" (and by extension, they are not considered minions).
FYI, this was from several years ago, and I'm pretty darn sure it's been verified that the +damage also does NOT work with traps.