That color sensor might work instead of a spectrometer with that liquid test kit, it just probably wouldn’t be as accurate as a spectrometer.
The test strips actually change different colors when you dip them in the water, but a liquid test kit like that will technically only turn the water one color when ammonia is present. The color doesn’t actually change at higher ammonia concentrations, it’s just more saturated so it appears to humans as darker. You would have to test that sensor to see how well it can determine different saturations of the same color. The method that test uses is supposed to turn the water green, so you might be able to put something red behind the sample so the sensor could be able to see a transition from red to yellow to green at higher ammonia concentrations.
For drawing the sample and adding the reagent you could use something like this: https://community.spark.io/t/spark-spark-relay-controlled-peristaltic-dosing-pumps/4888