Charms, like items, are color coded based on their rarity. While rarity can be a good guideline to the power of a charm, it does not mean you should forget about the more common charms. Even the most common vendor charms can have their place in a deck, depending on your strategy.
(The following examples, where applicable, are all Twinblades charms. The level range is 40-41)
These charms are white-ish in color, and are available in the charm shops in all the towns. While they are weaker than more rare charms of similar levels, they have no rank requirement and lower stat requirements, making them useful for raising weapon skills. Depending on your strategy, you may also opt to use a few vendor charms as filler.
During the 25th Contest, this color was used for Common charms.
Example: Mithril Short Swords, 40 Melee (45% P)
These orange-colored charms can be made in certain craft shops, and will be quite common in competitive gears. While craft charms themselves are common, the rarity of the materials used to make them can make some crafts very powerful for their level. All PvP charms are crafts, representing the straight damage/protection options for each weapon and armor type along with some moves with extra effects. Most of the charms in the auction house are of this rarity.
Example: Glorious Blades, 49 Melee (38% P)
Treasure charms are green in color, and are gotten from non-boss Dungeon fights. Treasure charms are designed to be slightly stronger than similar-level crafts. Treasure charms tend to have many combinations of runes, and may have some interesting effects as a result. Treasure charms may be themed towards whatever dungeon you're in (For example, many of the charms in Ghost Mine are Spirit based, to go with the Spirit damage theme of the dungeon).
During the 25th Contest, this color was used for Uncommon charms.
Example: The Skyclaw, 36 Melee (45% P), additional 39 Magic (25% P) if your last charm was Lightning. Follows the dungeon's theme of extra effects depending on charms played.
Gotten from boss fights in the Dungeons of Estiah, these blue charms tend to be some of the strongest charms you can use. Like Treasure charms, Rare charms may follow the theme of the dungeon. Some of the stronger Rare charms can be used for many levels. Complementing these charms may be rare boss-specific items that you can use to make powerful craft charms. This rank is also the most versatile of all the different charm colors: While there may be skill requirements, Rare charms will usually have minimal stat requirements, thus allowing them to be used by most if not all characters.
During the 25th Contest, this color was used for Rare charms.
Example: Obstination, +61 to next Melee and to next Magic if any runes from the last two charms played are the same.
Red charms that are unique to a particular class. For the first two tiers, they are bought like vendor charms, with certain class-specific effects that get stronger as you use higher-leveled charms, but in the third tier they are crafted using boss crafts, and have many unique effects. These charms are what usually defines the player's decks in the later levels.
Example: Assassin's Sigil, 49 Melee (45% P), Vanish
Purple colored, extremely rare, extremely powerful. Epics have special flavor text with them, and are heavily themed to where they drop from. These charms typically found at the end of a questline or in specific dungeons as a very rare drop. Epics can also be awarded from events, usually as a powered up version of certain charms like the epics awarded from the 10th Contest. Epics are like rare charms in that stat requirements are either non-existent or lessened, allowing you to use most epics that you obtain with relative ease.
During the 25th Contest, this color was used for Super-Rare charms.