I don't know if this will help you, but I usually use chocolate brown fondant, or knead in cocoa powder along with the brown colouring, I find that using too much colour makes the fondant too sticky and un-manageable, but the cocoa powder firms it up, while contributing colour and a yummy taste!
Also, Wilton sells a four-color package that includes dark brown.
I agree about using chocolate fondant then adding brown coloring and/or cocoa powder. Whatever you do, DO NOT add a touch of black coloring. I relented to that temptation one time and ended up with a pile of sludge that looked like something my cat throws up.
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