You have to apply this script to usual candles and not Heikin-Ashi!!!
Background color indicates HA bar color. On change you'll see green/red arrows and alert will be fired.
Also you can find in the code calculations of all HA OHLC values.
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.