I am not convinced all suggestions above provide are a one for all solution. My scenario is I have a grouping that has has multiple columns. I could not use the agreed solution RunningValue because I don't have a single column to use in the function unless I combine (say a computed column) them all to make single unique column.
I could not use the VBA code function as is for the same reason and I had to use the same value across multiple columns and multiple properties for that matter unless I use some other kind of smarts where if I knew the number of uses (say N columns * M properties) then I could only update the RowNumber on every NxM calls however, I could not see any count columns function so if I added a column I would also need to increase my N constant. I also did not want to add a new column as also suggested to my grouping as I could not figure out how to hide it and I could not write a vba system where I could call function A that returns nothing but updates the value (i.e. called only once per group row) then call another function GetRowNumber which simply returns the rownumber variable because the colouring was done before the call so I always had one column out of sync to the rest.
My only other 2 solutions I could think of is put the combined column as mentioned earlier in the query itself or use DENSE_RANK and sort on all group columns, i.e.
DENSE_RANK() OVER (ORDER BY GroupCol1, GroupCol2, ...) AS RowNumber