Depending on how big your collection is right now, I'd suggest switching to new gray ("bley"). The parts are cheaper, more readily available, come in more shapes/sizes, and won't fade as easily).
You can also mix in your old gray with new gray to get a mottled look. For example, here is a WIP picture of a large display I made for BrickCon last year using both old and new gray (if you look closely, you can see a slight color variation between the light gray and light bley):
Similar points apply to brown vs reddish brown.