If Sellers are more, you will red only, whereas if buyers are more, you will selling volumes and then extra green over it which shows how many buyers are there in the market.
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.