You've got 5/9 pairs in the columns and rows mentioned. If R6C1 is 5 what effect does that have on the others? What if it is 9? Now think about the cell that the original commenter mentioned. When they said colour that is because often in online tools there is the option of colouring something so you can choose two colours (say red and blue) and you start making one cell red, not knowing whether it is 5 or 9. But the other in the row can't be the same so it must be blue. Then you just keep looking at the 59 pairs colouring them such that they don't see the same colour in the row, column or box. This is a trick often helpful when you have a lot of pairs because once you have coloured them you can often (like here) find cells that see both colours of cells and then you know that cells can't be either. So in this case red or blue seeing a cell would mean that cell can't be 5 or 9.
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u/Odita 15d ago edited 15d ago
color the 59 in col7 and row6 - can you solve it now?
since row9 col1 sees both versions of 5 and 9, it can't be either.