I've tried a few ways.
Robert's Brick Paste - Its talc or some kind of powder in suspension. You brush it on, let it dry and wipe off as much as you want. Spray with dullcote to set. The dullcote tends to dilute it a little more, so multiple coats may be necessary.
Acrylic Paint - the cheap stuff from walmart.... white, light grey, light tan, whatever you want to achieve for a look. Thin it with isopropyl alcohol, brush it on and wipe it off as you go. If you dont like it, alcohol on a rag will remove more even after it dries. Set with dullcote, doesnt wash it out like the Roberts.
After I do it in white, I like to go back over it in spots with something dark, either an asphalt grey or black. Some people black wash the whole thing, it depends how dirty you want it.
Its very hard to mess up and not reverse it.... just make sure you like it before you get the dullcote out.