Q: Can there be a sixth color?

A: Short answer: No. Long answer: Not really.

Q: Why not?
A: There's not enough unexplored design space available to support an entire color. Throughout Magic's 20+ year history, the entire game was designed under the assumption that there would only ever be five colors, no more, no less. Nearly everything that can be done has been done, and can be found somewhere on the color pie. Additionally, you'd have to re-balance the entire color pie, since a new color would invalidate most of the different relations the colors already have with each other. Imagine getting rid of hundreds of enemy-hate cards, off-color cards, and multicolored cards. The entire game would change.

Q: What about making new space for a new color?
A: The problem with that is that it would quickly run into issues with how the game operates as a whole. Making a new color that does something like manipulate exile or the command zone, is just asking for trouble, especially since these zones were specifically made to be very hard to interact with.

Q: What about just taking elements from other colors and use them in the new color?
A: Because it doesn't add anything new to the game. In fact, during the Time Spiral block, Wizards even played with the idea of introducing a new color, Purple, to the game, with the idea that they could either keep it if it did well, or ditch it if it did poorly. As you might have guessed, it didn't even get through the development stages. Playtesters were just disappointed when they realized that all Purple did was take elements from other colors rather than add anything new or unique to the game.