Skills are rolled in addition to the color value chosen to accomplish an action, whether it be casting a spell or talking to get out of a situation. The value in question depends on how the player decides to apply it. A player with a high passion value may fare better trying to use their emotion to use a skill than if they try reason another way so they would use their red mana to supplement their skill.
The two combat skills, Spellcraft and Physical, can use mana from any and/or all of the colors.
The use and understanding of spells and magic.
The use of the body to move and act strenuously.
Interactions with people.
Instilling loyalty and command.
Senses both mental and bodily.
Ability to learn, as well as knowledge of history.
Sneaking, larceny, general thievery type skills.
Knowing how to live in dangerous locations and travel.
Viewing the Aether and feel of power around you.
Proper plane jumping.
Exerting control over summoned creatures.
Use and craft of Artifacts