Agree with above. Also, the "light" that you use to read the colour is very important. Always use natural diffused daylight, which means daylight but not direct sunlight. Natural daylight will render a true colour rendition which I have found makes it much easier to read the result. All artificial light, whether incandescent, fluorescent, or LED will distort some colours, more or less depending, and are unreliable for colour tests.
Most tests are best read with the phial held in front of the white card, and next to the colours on the card. The GH and KH tests of API can sometimes be difficult to read but with these it is best to look vertically down into the tube over a white background (the tube standing on white), you can readily detect when it first changes from orange to green, or whatever.
