Each notorious monster in abyssea can be triggered one of three ways: red, blue, yellow (grellow it's called by some, because it looks greenish-yellow). Each trigger is completed by a particular special action. Blue and red are triggered by particular weapon skills. Yellow is triggered by a particular magic spell: BLM, BLU, BRD, NIN. Each trigger effects something relating to a drop. Yellow increases the likelihood of AF3 items. Blue increases the normal items (weapons, armor, items) and Red increases the likelihood of a key item to pop another NM or atma/abyssite.
To sum it up: TRIGGERS R GUD!
The reason BLM/BRD is mentioned specifically is because BLM already can trigger quite a few but each trigger has a corresponding reason for appearing. Yellow specifically is affected by the day of the week before, during and after you pop the NM. Popping on Iceday will ensure the trigger is wind, ice, or lightning. So therefore BLM can trigger with Thunder, blizzard, aero III/IV. Thundaga, aeroga, blizzaga III, and burst, freeze, and tornado. By subbing BRD, it also allows the possibility of the one trigger from the job, increasing the possiblity of triggering by three: Water, earth, and wind threnody.
TL:DR
http://wiki.ffxiclopedia.org/wiki/Abyssea#Yellow_Weakness
http://wiki.ffxiclopedia.org/wiki/Abyssea#Red_Weakness
http://wiki.ffxiclopedia.org/wiki/Abyssea#Blue_Weakness
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