Starting with http://bl.ocks.org/mbostock/4060606
Step 1. Change
.defer(d3.json, "/mbostock/raw/4090846/us.json")
to
.defer(d3.json, "https://gist.githubusercontent.com/mbostock/4090846/raw/us.json")
Step 2. Add the following code to color one of the counties red
d3.selectAll(".counties path")
.style("fill", function(d) { return (d.id == 30041) ? "red" : null; });