What does the colored damage numbers mean?

These are visual indicators that will tell you if you're dealing effective damage (+50%), if you're the enemy is countering/resisting your attack (-25% damage), or if you're just doing normal damage. If the damage has purple numbers, it means that the attack deals reduced damage against the target. If the numbers appear as yellow, it means that the attack is dealing increased damage against the target. Neutral damage will appear as white numbers. (Like fire versus wind or water vs rock) Your equipped fairies' elemental types will affect these numbers.