I'm currently assuming anything that's the right element has a chance to proc Grellow. Potentially excluding Tier 1/2 nukes.
Lightsday - Bard and White Mage mostly. RDM for Dia III maybe.
Darksday - Dark Knight mostly. Red Mage for Bio III maybe.
Elemental days - Black mage for all the nukes, Scholar for helices, Anything /NIN works for most NIN stuff, but need a NIN for San nukes and stuff
Blue mage - Everywhere, everything, always. Mage of last resort. I'd bring one to every T2 pop NM though, if you want to be sure to hit the grellow !! for those AF3+2 items. Realistically, they should be able to set all the magical "elemental" spells of adjacent days in 1-2 "setting" sessions without issue.
If san nukes can trigger, someone needs to die.
Is there even a single damn legit report of blue magic proccing yellow? I highly, highly doubt they can since these add ons dont require TOAU, that they'd use spells from a job that you need TOAU to unlock.
San nukes have been reported twice (if Dia III procs idk why they wouldn't) and both Kirschy and a soloing BLU reported BLU magic proccing Yellow. Firespit and Magnetite Cloud were the two spells in question. Not much ambiguity there (soloing BLU can't proc with much else), but we can hope that it's only magic damage type BLU spells.
lacovie - freezebite - iceday 12:45
mictlantecuhtli - shadow of death - iceday 21:45
lacovie - tachi:koko - iceday 23:25
glavoid - thunder4 - lightningsday 19:20
mictlantecuhtli - freezebite - lightsday 11:25
myrmecoleon - cross reaper - lightsday 21:30
cep-kamuy
darksday: banish 2
firesday: bio 2
Apart from Thorny's assertion that it's true, I haven't seen any indication that merit spells necessarily proc at the same time their non-merit versions do. At the moment I'm assuming that SE just picks two spells at random from the appropriate 3 elements, which would explain why people are able to proc Grellow fairly consistently even without a BLU or using NIN nukes. On that note though, knowing it's limited down to 3 elements really helps BLU, as it reduces the number of times they have to set spells pretty dramatically.
Athamas - Blizzard3 - Iceday
Wherwetice - Stone4 - Earthday
Not that it's groundbreaking news at this point, I'd imagine.
Got a Firaga III proc on Wherwetrice, firesday.
No other magic was being casted at the time, not sure of people were still skeptical about -aga triggers.
[16:13:03]Starstrukk starts casting Firaga III on the Wherwetrice.
[16:13:08]Your attack staggers the fiend!
[16:13:08]The fiend is unable to cast magic.
[16:13:12]<Starstrukk> got it
[16:13:12]Starstrukk casts Firaga III.
[16:13:12]The Wherwetrice takes 1222 points of damage.
[16:13:13]Starstrukk's Stoneskin effect wears off.
[16:13:13]The Wherwetrice hits Starstrukk for 0 points of damage.
[16:13:14]The Wherwetrice hits Starstrukk for 57 points of damage.
Popped a Cirein-Croin on iceday, proc'd green/yellow !! with blizzard 4
Drops
well we got grellow with magic hammer on khalamari a few times , cant remember what day/element .
Popped 11 T2 nms in Coast last night, 9 of which we were able to proc Grellow. Every single Grellow was procd with spells of day or corresponding days. Which are now the only spells we use to try and proc grellow, could chalk up the 2 misses to human error or lack of blu mage/san nukes. All grellows gave 3/1 of the upgrade items except for a Sobek in which we procd Grellow first then couple minutes later, after we killed the mimic 1 of our members accidentaly popped >.>, we procd a red!! for atma and were rewarded with 2/1. Is 3/1 items not 100% from grellow? or does getting another !! of another type weaken the first?
On 1 of the dragons we got Blue!! right away then Grellow shortly after but didnt get the Rare drop from him, which i was hopeing Blue!! makes rare drop 100% but i guess not. Considering how completely random Blue!! appears to be.
Also Red !! =100% KI drop is the sexiest thing ever, went 12/12 KIs with red!!.