They're reusable so you don't need to worry about using it up. :)
From the FAQ (link at bottom of page, below footer)
What does a spell do.
A new Spell color comes out every month. A Spell has the ability to change one of your previous special item into a different color than its 3 default colors. All Donation Items, Coin Items, and some Limited Items have the ability to change colors using the Monthly Spell.
To use a spell, go to your inventory, then click on the desired item. Under the 3 default colors will appear available spells. Click the spell color box you'd like to use and it will change your item color accordingly. The item and spell will be added to your equip list. To remove the spell, either click on the item or spell in your equip list, they will unequip both at the same time.
The spell is not a consumable item and can be reused as many times as you wish. However, you can only use one spell per item at a time. If you would like to apply the spell on several items at once, you will need several spells.

by
shmabec » Wed Aug 01, 2012 12:30 pm