don't do this! First, you do not lightly paint brick it soaks up paint rapidly. Its OK to completely paint the brick, but your idea just will not turn out as you wish.
Suggestion, I cannot visualize grey siding with brown brick at all anyway. Sounds like the siding SHOULD have been a light earthtone of some sort, but not grey. I would not mess with the brick or consider painting over the siding, but to find some accent piece or even the shutters or something that I could use to TIE in the colors to make them more of a curb appeal.