[@Light]

Oh, and I forgot to answer the other part of your question - Yes, natural affinities would only be tied to the four Elemental Planes. To harness magic from the Abyss, one must plunge their mind into the farthest depths of their own inner darkness, which requires training and meditation, unless they've experienced some horrible trauma that caused them to draw upon the Abyss subconsciously. Conversely, to reach the Cosmic Void, one must attain a keen mind through rigorous emotional and academic self-discipline. 

And to make things easier, we'll just say Raw Mana is an invisible energy that doesn't take form or color until it is filtered into the Mortal Planes through the Magi channeling it. There can be some abilities known as [i][b]Universal Magic[/b][/i] that just use Raw Mana regardless of what Plane it comes from, which any Magi can easily use. The easier abilities from this would include stuff like magical blasts without a particular element and force/sound effects, while more advanced applications can be telepathy, greater telekinesis, etc. 

To give everyone creative freedom for how their magic looks, we'll just say the color of their summoned Mana and the subsequent magical effects are purely visual flavoring, like the color of one's hair or clothing. That means if someone is attune to the Plane of Fire, for instance, their flames can be any color they want.