Zapins, 0 ppm iron will certainly result in no discoloration (/water will stay clear)
The test kit will measure iron levels from 0,008 ppm to 0,8ppm
Attached is the colour chart:
The colour chart itself is (like you see) tiny and a little bit poor, but we need to consider that this company has also a primary focus on saling their photometers
for aquatic tanks.
So if you want to use the iron kit without photometer, it would be a good idea to make first some reference iron solutions and to print after doing so yours own colour chart ( for easier & better ppm reading )
The kit costs 17Euro and you can use it for 40 tests.
It is not that cheap test kit, but I think the price is fairly ok if it can measure all chelated iron.
(iron II and III)