Hmm, maybe regular brown acrylic could work as well? Depending on the brand of course, I've had burnt umber paint that was really thick and lumpy, it looked exactly like mud.
I don't remember the brand but it was artist grade paint. In my experience that colour (burnt umber) and darker browns and blacks in general are often of this thick, lumpy consistency rather than smooth. And if yours is smooth, you can always follow PKW's advice and mix in some sand and other stuff.