I think Sarah's observations are accurate and her explanation is close enough for jazz.
Prime breaks chloramine into chlorine and ammonia. It detoxifies (encapsulates?) the chlorine, then converts the ammonia to ammonium, leaving a slight, green ammonia/um reading.
Thanks for the overview on Seachem's Alert product, Sarah. Glad it's functionally accurate.
The ratio of free ammonia (NH3) to ammonium (NH4) is pH dependent. The higher the pH, the higher the free ammonia. The lower the pH the more ammonium. It's also effected by temperature, but less so.
Here's a chart you can use to determine how much of each is present in your tank.
Sorry I can't find the programmable version, but this ought to do.