Most brush pens only come in black. Occasionally, they also come in grey, but very rarely are there any additional colors.
My favourite brush pen, the Pentel pocket brush, only has black cartridge inks available. A few years ago, when I picked up a new Pentel pocket brush for a fresh tip, I decided to syringe-fill the old one with grey ink (specifically J. Herbin’s Gris Nuage). This was a great decision, and I’ve really enjoyed being able to use my fav pen to add value to drawings in addition to making the lines.
More recently, Pentel decided to release five limited edition versions of the pocket brush with different colored pen bodies. Alas, they’ve found my weakness… collectibles and colors.