django_tables2 allows for you to specify an alternative custom template for outputting your tables. Take a copy of django_tables2 / templates / django_tables2 / table.html
and rename it e.g. table_pass_rate.html
and enter your tag on line 29:
{% pass_rate_color cell %}
Now when generating the table use:
{% render_table table "table_pass_rate.html" %}
See the django_tables2 code for tags and the template for more info.