Continuous testing after BFP .. question


I’m seeing some posts that once they get BFP they are still testing to see if the line is progressively getting darker. My question is that how can that be accurate when the test itself turns much darker after couple of hours. So what’s the bench mark ? How can you really tell if the line is darker each day you test ? And with that , are you testing with the exact same type of test ? Pink line vs blue line ? I feel like I’m freaking out by doing that Bc today my line looked lighter .. according to the app I’m 5 weeks today ... any suggestions please ?????