Here is another approach to the problem using custom fills.
All columns are on the same axis. With overlapp set to 100.
I created to grouped shapes to use as the fill.
The group is a rectangle with no border or fill. Make this roughly the width of a column in the chart.
The second shape is a thinner rectangle coloured as appropriate.
With the 2 shapes grouped you can use copy and paste to fill the column series.
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