How do I make a brown buttercream icing, when I use a brown icing color (Wilton), it turns out a tan color, even if I add Hershey's coccoa mix.

You can use melted choc semie sweet or bittersweet. I use 3 squares per recipe of buttercream. It will get stiff, you will need to re whip it.

Maybe try adding some black in the with brown. Or maybe try buying Americolor's Chocolate Brown- their colors are more true and use less to get the desired shade.
I give Americolor a

Only add the chocolate if you're wanting chocolate icing. As most here know, I steer clear of Wilton coloring agents (and loudly). Once again, this post proves exactly how ineffective and costly that brand is.

Do your best to get ChefMaster liqua-gels or Americolor. Just a drop or so in a cup of icing and you have the color you want. I even add it to my chocolate icing to get that deep brown fudge color.

Heehee, what Dawn, you don't like the Wilton colours, really? Haha!
Ok, enough fun, I have to say that I agree on this particular colour, there is no way in heaven to get a true brown using the Wilton brown unless you do add cocoa or melted and chocolate, it just doesn't do it on its own! Regardless of how much of the Wilton brown you use, it just won't happen. Now I do find that you can get any other colour to work with the Wilton colours, although, for the darker colours you do use a lot. The advantage with Wilton colours, is that you don't get as much effect from the liquid as they are way less liquid than most other brands. So when you are mixing up red fondant for example, you use a heck of a lot of colour, but less icing sugar to make up for it being sticky. Whereas the others have a lot more liquid and therefore colour faster.
But the brown, well it is a big problem!
Hugs Squirrelly Cakes