private String getUnusedColor(Map<String,Boolean> colorsUsed){
        for(String color : colorsUsed.keySet()){
            if(colorsUsed.get(color).equals(false)){
                return color;
            }
        }
        throw new RuntimeException();
    }
http://www.geeksforgeeks.org/graph-coloring-set-2-greedy-algorithm/
src/com/interview/graph/GraphColoring.java