A selector uses the Color State List Resource. I'm not sure there is, in the list of states, the expected behaviour. However, you can achieve this with a click listener event instead of using a selector:
// init a boolean
private boolean isPressed = false;
// click listener event
btn_wb.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
// change the background once
if(!isPressed) {
v.setBackgroundResource(R.drawable.web_wl);
isPressed = true;
}
}
});