Some of the colors are pretty close. Try putting the test tube up against a white piece of paper, that may help or near a window. The nitrates should stay pretty low if you keep up with the gravel cleaning along with a minimum 50% water change per week.
10-20 isn't very bad but if you do see it closer in color to the 20 or above then doing a quick partial water change will help bring it down. The Api Master test kit that you're using is a great kit. Besides having lots of live plants - water changes are the only way to bring down nitrates