What do the different colors mean in regards to stock volume?

Every stock chart out there has volume at the bottom. I have a good idea what the volume means and what it stands for, but I am confused on the colors of the volume bars. What is the difference between a red/green/black bar. I assumed that a green bar meant volume was increasing and red meant that it is... show more Every stock chart out there has volume at the bottom. I have a good idea what the volume means and what it stands for, but I am confused on the colors of the volume bars. What is the difference between a red/green/black bar. I assumed that a green bar meant volume was increasing and red meant that it is decreasing, is that correct? And what does a black bar mean? Thanks,
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