I’ve had a big batch of rehairs to do lately. You can see that the coloured hair is quite popular. I dye the hair myself with bright hair dye (it works well on the horse hair). Kids love it, and it helps teachers too. They can explain where to play on the bow by referencing the coloured area. Since rainbow is the most popular, a teacher can say “play from red to purple”, or, “only in the green zone”. The hair responds well to the dye and it doesn’t effect how the hair plays on the strings.