Can you be a Tae Kwon Do instructor even if you are only a Hi-red belter and not yet a blackbelter?

Officially, no, but unofficially - it happens in many schools.

There is a difference between being a teacher of something, and being a certified instructor. In Taekwondo, many high ranking students help teach, but are merely on an instructorship program to learn how to teach, and are not yet certified. Each Taekwondo organization has different rules and requirements for what it takes to become an instructor. Technically, instructor certification is separate from rank promotions, although it is generally a stipulation that a student must obtain a minimum rank before being permitted to teach.

While anyone can open a school in many countries (except Korea), and claim to be a teacher without proper training or credentials, any legitimate school of Taekwondo would follow the guidelines set forth by either the Kukkiwon in Korea, or a major International governing body for Taekwondo instruction.

High color belt grades such as 3rd geup, 2nd geup, and 1st geup, are often considered senior students, or "class leaders" and may help guide students in various activities during a class, under the direct supervision of a certified instructor. They lead exercises, and make corrections on techniques that have already been taught to the student by a certified Instructor (Gyosanim). The certified title of "Assistant Instructor" (Jokyonim), is usually required to be at least a 1st Dan Black Belt, however some schools grant titles of assistant instructor to the brown or red belts.

A fully certified Instructor is one who can lead a class from start to finish, and is typically a 2nd Dan or higher, although some schools allow 1st Dan to teach classes solo. A senior instructor is usually 3rd Dan and above, and is highest ranking instructor in a particular Dojang (school). A school owner is typically referred to as a "sabeomnim" ("Sabeom" with the honorific "nim" for respect from a junior). Ultimately, it is up to the Sabeomnim as to who may teach in his/her school, but there are often association guidelines to be met, and can be read about through the organizations official guide book or syllabus.