Can someone please explain ?

Do you get evaps on both negative and faint line tests ? I've had negatives what feels like forever and I've gone back to them later and never seen an evap. Or even the slightest sign of anything.But i have what i think is a vfl (unfortunately blue dye) and it got darker its now very noticeable. It also still has colour. I know you shouldn't look after 10 mins so I'm going by the picture i took in the time limit. I am going to test again maybe tommorow or friday with a pink dye but i just didn't understand the evap thing.