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!