Red for strength, Green for accuracy, Shards for defence.
When a set of colours is bestowed to a player, that player can press 'H' to see the value, known as roll, of the colour, e.g:
Magically accurate (25)
A higher number indicates a better roll and increased proficiency.
Ultimately the best cast of colours will come from staff which are often rewarded to players for RP's or as a prize in competitions.
Following this, the best possible cast of colours from players will be from Shamans. However, a level 100 wizard will be able to cast higher rolls than a level 5 Shaman, this is due to the Wizard having access to higher portals of learning.