Colour Sudoku is another variant of Sudoku and can be found online via Colour Sudoku. Also an explanation how Colour Sudoku should be solved can be found on their website.
It’s very simple: In Colour Sudoku, instead of filling in the numbers 1 to 9 in each of the 3 by 3 blocks, you must enter 1 to 9 in the 9 boxes of each colour – for example there are nine yellow boxes. This means you have to look all over the Sudoku to see where the numbers fit in stead of each row or column.
I think the only downside is that you have to print them on a colour printer because you can not solve them online (yet) and the colours are too close to really see the diffence in a blink. Hopefully they solve these two problems soon. Happy Puzzling!