The classes are more behavioral guidelines than strict rules - similar to good/evil/chaotic/lawful alignments. All classes can earn mana of all colors if they perform the appropriate actions, and this can gain them abilities from other colors, and if unusual circumstances occur they can even shift their color over time. Purists will tend to gain presence faster though, and changing color becomes more difficult as the spirit's presence increases / they become more famous.
Simple example: All spirits gain Red mana by demonstrating their Power, and this gives them more powerful "active" abilities. A "monster" can use their abilities for benevolent causes, and this will gain them Green mana and eventually Green abilities if they do so enough, effectively shifting them to a Guardian (since a Guardian is basically just a benevolent Monster). But if they just wreck stuff whenever they can, they will demonstrate their power more frequently and thereby gain Red mana faster than if they limit themselves to times when it would be helpful.
Also worth noting that there's no real "evil" color alignment. Demons, monsters and tricksters may be anywhere from neutral to evil. But it is easier to find opportunities to harm than it is to find opportunities to help.
For people confused about the Mana system, basically the more "supernatural" something is the more mana it costs. A person getting an idea, or the wind jostling things over? Perfectly natural, and therefore without cost. Fireballs spontaneously appearing in a forest, or a perfectly sane person hearing voices? Still possible, but less so. Proper magic will be more costly, but once you have hundreds of mana to spare it won't be such a huge issue. When there are exceptions to this rule, there is usually a reason behind it.
Quill's a rather interesting concept, I'd probably consider him a Familiar at first since he seems to be equally balanced between Power and Wisdom, and is probably benevolent... but I can classify him as a Guide if that's what you're going for. There is no relationship between a person's soul color and the color of a spirit attached to them. Actually, don't place too much weight on someone's text color, it's mainly just there to make reading long walls of text easier.