Solutions provided in this forum are meant to apply to the broadest range of traders. We are here to provide solutions to the most common questions faced by traders. As such, we try to avoid using fixed values for comparison because fixed values do not have broad application.
For example your request for a fixed value of 500k shares would be meaningful for a very limited number of stocks and completely useless for the vast majority of stocks. So in my solution I am replacing your fixed value with a comparison between current volume and average volume. When volume is above average the column is green. When volume is below average the column is red:
input length = 14;
plot data = volume > Average(volume, length);
AssignBackgroundColor(if data then Color.GREEN else Color.RED);