Any craft paint, $1.00 at any craft or dollar store. Also Walmart carries them. Or any acrylic paint for models. Sometimes you have to rub off any excess paint that stays on outside the letters.
I've been wanting to install a knot on this Navy Brass handle I got from @Douglas Carey for a while, but I wasn't sure what knot to use. After I got another Surrey brush in a lot I recently bought with a green handle, you were right @Enrico, I decided to use it's knot as I had another green one and it's NOS. I always considered Surrey boar knots as some of the finest made. This one didn't seem broken in yet and would fit the handle nicely...............