For a person who wears a v-neck dobok which is based on Won, Bang, Gak, practices Poomsae which is based on Won, Bang, Gak, it is philosophically incorrect to have adults wear Poom belts, and for children to wear black belts. But anything goes in USA, and usually due to lack of understanding how integrated the Won, Bang, Gak theory is into every aspect of Taekwondo.
The official KTA/Kukkiwon belt groupings are three. Geup, Poom and Dan in accord with the samilshingo idea of Won, Bang and Gak.
The official belt colors of the KTA, and eventually the Kukkiwon were the colors of the 5 basic elements. White, Yellow, Blue, Red and Black.
Yellow, Blue and Red are the colors found in the Samtaegeuk symbol of Korea and is from the same Samilshingo idea of Won, Bang and Gak (heaven, earth and human)
White and Black represent the Um Yang (Yin Yang) and for Taekwondoin, represents the idea of "beginning/completion.
Wearing the Poom belt upside down, according to the Won, Bang Gak theory it is based on, is like wearing dobok pants in place of the top, wearing the top for pants, and tying the belt from the top of your head to your crotch.