The question of what toothbrush makes a good bouldering brush is a question which cannot be overlooked.
Metolius has that M16 brush which is supposedly used to maintain rifles by the Army... however< i find it is too thin to be a good brush for local conditions. Perhaps it works OK for the desert where you are brushing sand and chalk off holds, but for coarse granite and moss it is substandard.
For local conditions large surface area and stiff bristles are beneficial. Ideally the brush will complement the functionality of a wire brush. One caveat is that dental hygenists recommend soft bristles, so it would be a mistake to buy a super stiff brush for your teeth, planning to use it down the line as a boulder brush.
Summarizing all of the above, an old "Reach" or private-label toothbrush can be stolen out of the mug on the sink counter and converted into a bouldering toothbrush. Pick one with a bent handle rather than a straight shaft. Firmer bristles are better. Finally, don't plan on using it for occasional toothbrushing afterwards. chalk and moss are not suitable for introduction to a mouth. Once a bouldering brush, there will be no going back.
With all that in mind, you can now consider fashion. My bouldering brush is purple, which nicely accessorizes the Goth blackness of my bouldering purse and A5 flip flops.