Agreed with the above, in general I tend to favor the tests that provide a "count the drops" or "measure how much you have used" with a clear color change (basically, a titration).
And for that the Salifert and Elos tests are awesome.
When it comes to how deep the color is, sometimes it is just impossible to use on low values. That is why I went to use the electronic Hannah for phosphates, since the Salifert is using the same product, but it is almost impossible to read a difference "by eye" unless your phosphates are over the roof.
In the low values, the Hannah checker is just really good because as far as I understand, it is just an optical measure how the change in color, at levels that our eyes can hardly see. Just make sure you wipe the surface of the glass containers very carefully before each measure.