As previously mentioned, some Kukkiwon schools used the
red-belt-with-a-black-stripe for "junior black belts" who are not yet of age. When the student turns 15 they can trade-in their
red-belt-with-a-black-stripe for a conventional black belt.
Some other Kukkiwon-style schools insert a rank between 1st Gup and 1st Dan though, so you'll see even
adult students wearing the "bo-black"
red-belt-with-a-black-stripe. The school that my son and I attend is like that: the bo-black belt basically means you've learned all the Taegeuk forms but you haven't yet passed your 1st Dan test. Since dan testing happens only a couple of times a year at our school, it's not uncommon to spend 4-5-6 months wearing a bo-black belt. I've seen that at other schools as well. What purpose does this extra belt-level serve? At our school, there are some classes during the week that only the black belt students may attend; the bo-blacks are allowed to attend those classes as well.
The head of our school is a Kukkiwon 7th Dan from Korea who started studying taekwondo in 1979. Now what he told us just recently is that at the school
he attended wa-a-a-ay back in '79 in Korea, they only had three belt colors: white, brown, and black. Then as time went on, he said, his school started adding more colors (to better mark the students' progress).
In the past I've looked at a lot of websites of a lot of taekwondo schools, curious about all the different variations of the color schemes. The school my son and I attend is a large school (hundreds of students - we had one class recently with 90 students in the class, but that was as special occasion) so we tend to not use tape-tips for belt colors, because it makes it hard for the instructors to see
at-at-glance who should be working on what. I think that might actually be a trend at larger schools: using many different belt colors (i.e., without using tips) perhaps makes it a little easier to organize large classes.
From what I've been able to see here in the U.S. at least, there doesn't
seem to be any "most common" color scheme. There appear to be many variations, and I haven't been able to note that any particular scheme is substantially more common than any other. At least, that's been my observation.