Reading API tests

elos makes a good mag kit.

 

I hold my API vials a couple inches in front of the color chart. It looks much different when resting it against the card. Also being under yellow lights will affect the perceived color. To get a true accurate reading on each of the params, one would have to spend a ton of money on those probes and readers/controllers. These are good for approximate estimations and serve as an alarm when the levels are bad.
As far as Mg kits, I use Seachem. Reef Aquarium Test Kits: Seachem Reef Status Magnesium & Carbonate/Borate Alkalinity Test Kit