I'm not the autor of this script. The original ZoneTrade script http://codebase.mql4.com/en/code/8534 is developed by the Duke3D. The Heiken Ashi version of the ZoneTrade script is developed by the brother3th. Thanks to the autors of their work.
This script is based on this trading approach that integrates Acceleration/Deceleration Technical Indicator (AC) and Awesome Oscillator (AO) into the Heiken Ashi candlestick chart. In this way it is a lot easier to trade in the zone - the 4th dimension described by the Bill Williams in his book: New Trading Dimensions: How to Profit from Chaos in Stocks, Bonds, and Commodities (A Marketplace Book)
Accoding to the developer of the ZoneTrade indicator it is working in this way:
If the current bars of AC and AO are green, it shows that the zone is green.
If the current bars of АС and АО red, it shows that the zone is red.
If the bars of AC and AO are differently directed then the bar is colored grey (grey zone).