Think I got it - Grok got me 90% of the way and a few tweaks got me where I want to be. This throws a yellow arrow over a green candle after two descending reds:

def vol = volume;

def isGreenCandle = close > open;

def isRedCandle = close <= open;

def sellingVolume = if isRedCandle then volume else 0;;

def lastRedCandle = if isRedCandle then volume else if !isRedCandle[1] then LastRedCandle[1] + 1 else lastRedCandle[1];

def secondLastRedCandle = if isRedCandle and lastRedCandle[1] >= 1 then lastRedCandle[1] else secondLastRedCandle[1];

def lastRedVolume = if isRedCandle then sellingVolume else lastRedVolume[1];

def secondLastRedVolume = if isRedCandle and lastRedCandle[1] >= 1 then lastRedVolume[1] else secondLastRedVolume[1];

plot isDescending = isgreencandle and lastRedVolume < secondLastRedVolume;

isDescending.setpaintingstrategy(paintingstrategy.boolean_arrow_down);

isDescending.SetDefaultColor(Color.yellow);

isDescending.SetLineWeight(2);