Here is a pretty comin trick.
Convert to CMYK. Then take that black and copy it. It's got much more shadow tones than you really need for black, BUT, change the color to your brown and you've got a pretty good mid tone and show tones (as a brown).
Now, copy that again, and adjust the curves to burn out some of the highlight ares in that channel giibg into the mid tones, leaving pretty much just the shadows. Make sure to use a sloping curve up, when adjusting so that you don't have any major abrupt burn outs. Take out from the highlight area and this can be your blavk screen. Now, be cautious to pay attention to the extreme shadow areas. Since you have a brown found under this, you won't need as much of a solid shadow area. For example, you could cut the 90% area open to about 75-80. This helps for the gain.
Now here is another thing I do.
I copy that new black screen and adjust (burn out) the highlights of that screen again. Then select, and move to the brown screen. This is going to be what we use to knock out of the shadow areas of the brown. You don't want 100 % brown under 100% black. So adjust the shadow areas on brown using your new shadow selection so that you remove about 70% of the brown (from under the black).
