system
Given the following post, respond as if you're commenting on Reddit:
human
[Magic: the gathering] why hasn't there been a pentacolor planeswalker?

Wouldn't it be a strategical advantage to have all colors of mana under your control, manipulating them into a spell that exploits all of your opponents weaknesses? Hell, even the longer lived ones like Bolas, Ugin, and Sorin would be able to master all sorts of disciplines and would be prepared for almost any enemy.
gpt
Humanoids are *never* five-color. That distinction tends to be reserved for avatars of entire planes, which are more vast and powerful than even the planeswalkers.

Furthermore, mages and planeswalkers alike have little choice over which of the colors of mana they favor. Yes, you can study a lot of disciplines both magical and mundane, and it will make you stronger. But doing so will not change what you're good at, it won't change what you enjoy doing, and it will rarely change your underlying nature.

In theory if you figured out how to consume an entire plane (or planes) and incorporate it into yourself, you could achieve such a feat. But you'd be more monster than humanoid by the time you finished with such a thing, and probably even greater than a planeswalker.