Using conditional formatting to alternate the formatting of data by category

Question

I have a spreadsheet with a bunch of rows like this:

Name    |   ID    |   Category  |    Attributes...
-----------------------------------------------------         


        

Answer 1

Please select ColumnsA:D and HOME > Styles - Conditional Formatting, New Rule..., Use a formula to determine which cells to format and Format values where this formula is true::

=ISEVEN(SUMPRODUCT(1/COUNTIF($C$1:$C1,$C$1:$C1)))

Format..., select your choice of formatting (dark?), OK, OK.

For 'light' I have assumed that either no fill is light enough or, if not, you apply standard fill to suit (CF, where triggered, will override it).

The COUNTIF is a sequential counter that starts at 1 for each category. Dividing that into 1 and applying SUMPRODUCT generates a sequential counter by category (only changes when the category does). ISEVEN is applied to pick alternate categories. To investigate the detailed workings of formulae M$ has provided Evaluate Formula.