Combing an If Statement with Pagination
To get the pagination to work as expected, you need to incorporate the if into your craft.entries query. There may be a more elegant solution to do this, but looking at the database, the selected choices is saved as an array-like string:

["blue","red"]


This means that you can search for the value in the field like this:

craft.entries.section('eventRentals').rentalColor('*"red"*').relatedTo(category).limit(12).order('title')