How to play

The aim of Sudoku is to fill the grid with the digits 1 to 9 in such a way that each digit occurs exactly once in each row, column and region.

• To insert a digit into a square, move the mouse pointer over a square and use the keyboard to type a number into the square.
• To remove a digit, move the mouse pointer over a square and press the spacebar on the keyboard.

Colour sudoku adds another dimension to solving by assigning a colour to each digit. Squares containing a digit are coloured according to the digit's colour. Empty squares are coloured according to which digits are plausible taking account of all current entries in the square's row, column and region. The empty square's colour is the combination of the colours assigned to each plausible digit. This means that:

• Darker coloured empty squares imply fewer possibilities.
• An empty square that has the same colour as a completed square must contain the same digit.
• If a black square is encountered then a mistake has been made (assuming no digits are assigned to black).

The colours that are assigned to the digits can be changed from the panel on the right-hand side. Another strategy for solving is experimenting with the colour assigned to the digit.

Note that the 'plausible' digits are exactly those allowed by the basic rule. A little reasoning may reduce them (hopefully to one possibility). For example, in Puzzle 1 put the cursor over A7 and it shows 6, 8, 9 as plausible. But inspection of the 8's in regions1, 4 and 9 shows that 8 cannot occur anywhere else in region7 so it is the only possibility for A7.