Incantations and Occultism
First here’s the link to the main article about the Colors of Mana: http://wiki.mtgsalvation.com/article/Color_Pie
Especially note the tribes at the bottom of each color’s main article. These are some of the fantasy species associated with your colors.
Magic on Ravnica is not a single focus, it is a myriad of disciplines across the whole spectrum of possibility. Such as it is, it is impossible for any one person to study or practice more than a narrow band of magic.