She told her friend that her favorite colors _________ red and blue.

The correct choice is "are" because it agrees in number with the plural noun "colors." In English, when referring to multiple items or categories, the verb must also be in the plural form to maintain grammatical consistency. Since "colors" indicates more than one hue, the plural verb "are" is the appropriate choice to convey that her favorite colors include both red and blue. This construction accurately reflects the subject-verb agreement essential in standard English grammar.