function frame2time(clip c, int f)
    {
        rawtime = f/c.framerate
        hours   = rawtime/3600.0
        minutes = frac(hours)*60.0
        seconds = frac(minutes)*60.0
        time    =string(floor(hours),"%1.0f")+":"+string(floor(minutes),"%02.0f")+":"+string(seconds,"%05.2f")

        return time
    }

function prepareline(clip c, string s, int f)
    {
        result  = RT_StrReplace(s,"0:00:00.00",frame2time(c,f))
        result  = RT_StrReplace(result,"0:00:01.00",frame2time(c,f+round(c.framerate)))

        return result
    }

directshowsource("test_hibiki.mkv")

#Variables:
filename       = "subtitle.ass"
stringtemplate = "Dialogue: 0,0:00:00.00,0:00:01.00,DefaultStyle,,0000,0000,0000,,"

#Coordinates of the textbox for 720p
tbX      = 198
tbY      = 584
tbW      = 880
tbH      = 92

#Starting with a new line just to be sure.
writefilestart(filename,"chr(13)",append=true)

textbox  = crop(tbx,tby,tbw,tbh).coloryuv(autogain=true,autowhite=true ).levels(8,1,255-8,0,255,coring=false)
detected = false

temp=""" detected=true          \
    writefile(filename,"prepareline(last,stringtemplate,current_frame)",append=true) last"""
t="overlay(blankclip(width=tbw,height=tbh,color=$00FF00),x=tbx,y=tby,opacity=0.3 ) "
#^ indicates detection by placing green bar over the textbox when it occurs

scriptclip("""
 tal=textbox.rt_averageluma
 (tal<235 && tal>190)?           \
    eval(t+"(detected==false)?   \
        eval(temp):last          \
    "):                          \
    eval( "detected=false last")
""")

assumefps(270)    # You can make the script scan your video faster by raising the framerate here, if you're running it via player. It works at ~330 fps for me.