That 6 is red because it's wrong. Sudoku ninjas can turn off the highlighting of wrong guesses, of course.

Mighty Mighty's Sudoku is quite customizable. Players can choose timed or untimed games, decide whether incorrect answers should be highlighted, and even set the board's color scheme.

There's a free version so you can check out the gameplay, but the real customization options happen in the $3 paid app, which features more than 250 puzzles in four difficulty levels. There's no music, which means you can listen to your iPod without a hitch. And the portrait-view interface is serenly uncluttered.