Engine leaks detector, on a 2011 and 2013

After replaced engine on our 2011 Prius (230,000 miles), just to make sure the engine we bought was fine, I purchased a quick leaking test. Did the test and no bubbles and no color change. Did the test as well in our 2013 Prius (105,000 miles)but this time there was bubbles, but the color didn't change too. The test only mentions the change of color as indication of head gasket failure but nothing about bubbles or no bubbles. I repeated the test on the 2011 to make sure it was sealing completely the deposit, but again no bubbles or color change in fact I believe some kind of suction was there because when disconnected the cap I hear a pop sound. Any thoughts? Any idea why one has bubbles and not the other? Maybe I need to do something on the