Color Pie is the term used to describe the limitations of the five colors of magic, both for game balance and flavor reasons. If every aspect of magic is baked into a pie, each color gets only a slice of that pie.
A color can’t do everything well. For example, Green, the color of plants, can easily erode an unnatural artifact, but has problems killing creatures outside of head-to-head combat. On the other side, Black, the color of death, can effortlessly snuff out the life force of most living things, but struggles to deal with artifacts.
A balance has to be struck between using more colors to cover up their individual limitations, and using less colors to make it more likely to have the right lands for your spells.