I recently painted the brick exterior on an old house (it was previously painted a very tired brick red) to match an addition that was constructed of assorted colors of brick. I just used an acrylic latex paint and a small 3" roller and a deft touch to modulate the color and amount applied. It actually went very quickly. Just get a grey color paint and dilute it down with water or Floetrol. Experiment with different dilutions to see which works best for the effect you are trying to achieve. Keep a bucket of water and a sponge/rag handy to soften or remove paint as you go. A quart of paint should be plenty and start with a very weak diluted paint (more water than paint and gradually build up by adding a bit more paint each time, hopefully you'll get where you want to be. Good luck!