This question is a old one but it can help for others too.
Try this :
li.setBackgroundColor(getResources().getColor(R.color.blue));
or
li.setBackgroundColor(getResources().getColor(android.R.color.red));
or
li.setBackgroundColor(Color.rgb(226, 11, 11));
or
li.setBackgroundColor(Color.RED)