Col F contains a unique value, "C", or an empty cell.

Select all rows or upper left to lower right cells
Format/Conditional Format
change "Cell Value Is" to "Formula Is"
enter =$F1="C" then select Format/Patterns and a color

Click Add and change Condition 2 to "Formula Is"
enter =$F1="" and select Format/Patterns and the "No Fill" option

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