Pregmate evaps getting darker??

Lulu

So I bought a 50 pack of pregmate tests from target and I've been using them for the past 3 days. The evaps have been getting darker over the past two days but every single test has a thick evap line that shows up after 10-15 minutes. Do pregmate tests usually have evaps like this? What do you guys think? I'm 12 dpo btw. The line showed up probably after 10-15 minutes