The original title of your question has been changed. The title you entered "Need Help plotting down arrow on top of bar" could describe any one of a hundred different posts in this forum. We need to be more specific than this. A quick search of the forum will reveal many of examples showing "how to plot a down arrow on top of a bar".
You said you included an example but I did not see any attachments or screenshots included with your post. But I understand you would like to have a down arrow plotted on the first bar of each trading session but only if that bar is red (close < open).
def newDay = GetDay() <> GetDay()[1];
plot signal = newDay and close < open;
signal.SetPaintingStrategy(PaintingStrategy.BOOLEAN_ARROW_DOWN);
That code must be applied to a chart that has extended hours turned off.