The yellow side is to test for chlorine (disregard the "Br" nymbers, that's for bromine and I assume you don't use bromine)
So, you have two chlorine tests....what's up with that? Use the yellow side of the comparator and test daily at first and then you will let the testing taper off as you get familiar with your pool. That test is not particularly precise but it will always tell you if you have some chlorine. Sort of like reassurance.
Then, when you need to know exactly how much chlorine you have, use the FAS/DPD test that involves the powder and the Reagent R-0871. It is very, very accurate and used to shock, do an overnite FC loss test, and to accurately check the chlorine in the pool anytime you feel the need. Most folks perform all the tests once weekly until they get familiar with their pool and then less often after that.