Calculating percent of one field over the sum of another

I am trying to use multiple response data in a pivot table.

A survey was given to students asking them to check the colors they like. They could select as many as they wanted among Green, Blue, Red.

For example:

- 7 out of 7 (100%) females in School ABC selected Green.
- 5 out of 7 (71.4%) females in School ABC selected Blue.
- 2 out of 7 (28.6%) females in School ABC selected Red.
- similar data for the males in the school, and for students from other schools.

The data I have is aggregated like so:

School Gender StudentCount Color ColorCount
ABC    Female  7           Green         7
ABC    Female  7           Blue           5
ABC    Female  7           Red            2
ABC    Male    8           Green           8
ABC    Male    8           Blue            5
ABC    Male    8           Red             3
etc.

In the pivot table I can't figure out how to calculate the percents (7/7 = 100%, 5/7 = 71.4%), etc.

My Pivot table has StudentCount and ColorCount in the data area. My hope was to change the options for ColorCount to show data as the "% of" and select the StudentCount variable, but that's not an option.

Any suggestions? The data with pivot table is attached.
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