Usually Doboks for Gup grades will have a white collar, and Dan grades (black belts) will wear a black collar Dobok (not sure if there are any rules about this, but this is what I see as the common practice).
Are there practical ways to convert a white collar to a black collar?
This would allow newly promoted black belts to reuse their existing Doboks, that might still be in good shape.
Once I bought some black fabric and gave a tailor exact instructions on what to do, so that's doable. But that only saved a few dollars over ordering a ready-made black-trimmed top from the place my Grand Master uses for uniforms.
Our system does this, in that we have one collar that denotes Leadership students, which is a 1" wide ribbon sewn around the edging of the dobok (Not to include the bottom hem). For students that are in the instructor path, there are red/black (1st level certification), black/red/black (2nd level certification), and full black (fully certified instructor).
The type of ribbon that we use is called grosgrain ribbon, and is usually found commonly at fabric stores. As long as it matches your current ribbon, any local tailor/seamstress shop should easily be able to take off the old ribbon and put on the new one seamlessly (har har! :p). If your ribbon is a specialty type, inquire if you can get some of the ribbons by themselves and repeat the process with the seamstress.
The only difference being is if the new ribbon is narrower, you run the risk of the old seam pattern showing, but if your instructor is ok with that in class, go for it. I currently have class/school uniforms, and then a tournament/dress uniform that only gets worn for those occasions.
I don't see why you can't just use a fabric dye. Plenty of options for that.
Here's a type of dye I've seen in other threads that people use on their uniforms:
https://www.dharmatrading.com/dyes/dharma-fiber-reactive-procion-dyes.html?lnav=dyes.html
It's called "Dharma Fiber Reactive Procion Dye". They have it in multiple shades of black.
There are many choices for fabric dyes, though. That was just one. Amazon will have plenty. I think you could even use tie-dye dyes, if they have one in just black.
I've never done this, so it would be interesting to see how it turned out for you.
The main caveat to doing this is that you have to be careful not to let the dye spread across the line of the collar. To ensure that, I think you're going to have to paint it on by hand using a small paintbrush, at least at that line. The rest you can just soak a little (not too much, or it will bleed across the edge).
It's going to take some experimentation. Start on the back inside collar area, and close to the middle of the collar where a mistake won't matter too much. Put a dab on and leave it there for an hour to see how far it spreads. From that, you can get an idea of what it will do. Then do it for real. It might require multiple coats.
Alternatively - and I don't recommend this - see if you can use a Sharpie marker pen. Put a dab on somewhere, let it dry, come back and reapply a couple times, and wash your uniform a few times to see if stays on. I doubt it will produce a very deep black color, and it will probably fade over multiple washings.
Hope that helps.
