The prefilled numbers on the game board are white.  Your correct answers are yellow.  Your incorrect answers are red.

While playing a game there are some helpful features available on the row of the buttons at the bottom of the screen.

The Pause button stops the game and the timer, so you can take a break.  Just click Resume from the main menu to continue your current game.  You really can play this Sudoku game where ever and whenever you want. The Notes button allows you to enter numbers on the grid when you are not sure whether an answer is correct or not. There are two types of Notes technology available.  The Multiple Number Notes (on by default) allows you to note every number (1-9) in each square.  The Single Number Notes technology (can be turned on in the options window) is a Single Number Note shown using an italic font to distinguish them from normal answers. The Notes feature remains on until the next time you tap the Notes button. When the Notes feature is on, the Notes button is blue.  Noted numbers are not answers and must be replaced with your answers in order to solve the problem. The Clear button allows any square (except the originally prefilled numbers) to be emptied of its answer, note or hint. The Hint button will fill in one empty square with the correct answer if you have a square selected, otherwise it will choose one at random. Hints are always shown in green.

From the main menu, the Options screen can be selected which gives access to some other helpful features. The Display Timer option allows you to either show or hide the timer during gameplay.

The Show Incorrect option will change the way incorrect answers are displayed during gameplay.  When it is on, incorrect answers are shown in a red color.  When it is off, incorrect answers are not distinguished from correct answers.

The Smart Buttons option disables the number button when that number is completed on the game board.

The Smart Notes option removes the number from the notes in the box, column, and row that contains the cell with your correct answer.

Lastly, the color schemes option allows you to pick a different palette of colors for the game board and number selection buttons.