What type of report are you running? Are you talking about a list report on a dashboard. There are a ton of ways to do this. If you have Performance Analytics, that would be the best.
If your just creating reports and putting them on a dashboard, then I'm assuming you are using a list because you wanted to see a specific field change color under specific conditions.
You need to create a new "Field Style" https://xxx.service-now.com/nav_to.do?uri=/sys_ui_style_list.do
(Replace xxx with your instance name)
Table: project_task (or whatever your table name is)
Field: project_status (or whatever your field name is )
Then your "value" will be something like this (NOTE: UNTESTED and FIELD NAMES NOT ACCURATE):
Value:
javascript:(current.planned_end_date.getGlideObject() > current.actual_end_date.getGlideObject());
Background something like : background-color:tomato
