It took me much longer to find an answer for this than it should have, so I thought I'd share it with anyone interested here.
After you cast Libra, you can go into the Enemy Intel screen (RB or R1 during battle) to find out that enemy's weaknesses. The problem is, there's no legend to describe what the icons mean anywhere. Maybe it's common knowledge and has been used in past Final Fantasy games, I don't know, but I finally came across an accurate legend online:
Red (flame) - enemy is weak against fire
Light Blue (ice) - enemy is weak against ice
Yellow (lightning) - enemy is weak against lightning
Blue (water) - enemy is weak against water
Green (leaf) - enemy is weak against aero
Brown (earth) - enemy is weak against earth
White Sphere - enemy is weak against holy
Black sphere - enemy is weak against dark
Really comes in handy for certain bosses who can change their elemental weaknesses during battle.
