A "color pair" is exactly two colors
By pairing Teval with cards like The Gitrog Monster or Insidious Roots, you can create an engine that keeps your graveyard active and your board full of protective tokens, making your walkers nearly impossible to reach in combat
So, in a game of regular commander, with all of your opponents in the game, you could be casting four cards for free , including the one from your deck
These include two unique Ritual Summons, a variant of the Red-Eyes reliant on a Trap Card, and a Fusion Monster which looks nothing like its constituent parts
That kind of trade-off is usually worth it, especially since Ketramose is hard to remove and comes with menace and lifelink to help stabilize once its ready to attack