Colour Sudoku is the puzzle as normal Sudoku with an extra constraint. In a similar way to the rows, columns an boxes, the colours must also be uniquely 1 to 9. Each colour is in the same position in each 3x3 box, as you will notice with the central white cell, for example. All Sudoku strategies continue to apply - except the ones that rely on the deadly pattern used by Uniqueness strategies. Almost

The extra constraint does not add any extra complexity in the rows and columns - since a uniqueness constraint already exists, but it does effect cells diagonally. For example, B2, C5 and H8 are now connected and a conjugate pair may exist on those cells. So, like Sudoku X, the solver needs to be aware of all the new diagonal opportunities that are now possible.

Interestingly, this constraint forces the Sudoku puzzle to be 'perfect', that is, every number appears in a different position relative to each 3x3 box.

Note also that I have not completed the grading of this type of puzzle so I cannot offer a grading option in this version. The Solution Count program also needs updating to this will be available after the next update.

All feedback, comments, arguments, bug reports and strategy ideas are welcome. There is a new FEEDBACK form with a column displaying comments and questions. Many thanks to all the people who have done so and helped improve this solver.

Solver help
Use the "Import" button or type in a Sudoku puzzle in the small board.
You can also pick examples from the list above. Click on Take Step to step through the solution. Unknown squares are filled with 'candidates' - possible solutions.
Any cells that are reduced to one possible candidate are solved.

You can use the [<<] button to step back one go. Toggling between Take Step and [<<] helps you see the changes.

Pressing "Enter" on the keyboard after clicking on Take Step is a quick way to step through all strategies. Details of any solutions will be written out in the textbox below the big board. Strategies are ordered by complexity. Any that are successful returns the step-though to the start.

Click on the board to highlight sets of numbers. You can edit the Sudoku at any time - entering solutions in the small board or editing candidates (Toggle between highlighting and editing using the radio buttons at the top.)