Adding an alert condition to this trading view script

I am not very savvy on script but have been trying to understand and write an alert condition for a trading view indicator. I tried contacting the creator of the indicator but have heard nothing.

The script below changes from red to green and vice versa but i can't set an alert in tradingview. I'm hoping to add an alertcondition for when the indicator changes from red to green.

Any ideas on how to do this?

//@version=4
study("Donchian Trend Ribbon", precision = 0)
dlen = input(defval = 20, title = "Donchian Channel Period", minval = 10)

dchannel(len)=>
    float hh = highest(len)
    float ll = lowest(len)
    
    int trend = 0
    trend := close > hh[1] ? 1 : close < ll[1] ? -1 : nz(trend[1])
    
dchannelalt(len, maintrend)=>
    float hh = highest(len)
    float ll = lowest(len)
    
    int trend = 0
    trend := close > hh[1] ? 1 : close < ll[1] ? -1 : nz(trend[1])
    maintrend == 1  ? trend == 1  ? #00FF00ff :  #00FF009f :
     maintrend == -1 ? trend == -1 ? #FF0000ff :  #FF00009f :
     na

maintrend = dchannel(dlen)

plot( 5, color = dchannelalt(dlen - 0, maintrend), style = plot.style_columns, histbase= 0)
plot(10, color = dchannelalt(dlen - 1, maintrend), style = plot.style_columns, histbase= 5)
plot(15, color = dchannelalt(dlen - 2, maintrend), style = plot.style_columns, histbase=10)
plot(20, color = dchannelalt(dlen - 3, maintrend), style = plot.style_columns, histbase=15)
plot(25, color = dchannelalt(dlen - 4, maintrend), style = plot.style_columns, histbase=20)
plot(30, color = dchannelalt(dlen - 5, maintrend), style = plot.style_columns, histbase=25)
plot(35, color = dchannelalt(dlen - 6, maintrend), style = plot.style_columns, histbase=30)
plot(40, color = dchannelalt(dlen - 7, maintrend), style = plot.style_columns, histbase=35)
plot(45, color = dchannelalt(dlen - 8, maintrend), style = plot.style_columns, histbase=40)
plot(50, color = dchannelalt(dlen - 9, maintrend), style = plot.style_columns, histbase=45)