From Sudopedia, the free Sudoku reference guide

From Sudopedia

Even-Odd, or Odd-Even, is just like regular Sudoku but with shaded or colored squares which must contain either even or odd numbers according to the puzzle. For example, in an even-sudoku all shaded squares will only contain the numbers 2, 4, 6, or 8. This greatly decreases the possibilities and makes eliminating candidates easier.

This variant can be generalized further by having multiple colors, with each colour indicating the possible set of digits that can go into the cell.

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• Sudoku Variations

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