How you measure the colour depends on the test kit.
API you hold the tube up to the side of the card and look at the side of the tube, JBL you sit the tube on top of the card and look down, Salifert is the from the top as well, and Sera I cannot tell you because I only have a KH kit and all you are looking for is a colour change, but I suspect they are from the top as well because the diameter of the tube/test vial is larger giving a less concentrated view, whereas the API ones are narrower than a standard test tube.