You could use the paste. The paste would require smearing inside the vent more than the gel. Hence, another reason for the rubber gloves!
What you want is a high color concentration for lasting effect. The liquid from the grocery store would probably work, but it is rather dilute, so would not show up as well as the paste or gel. I use pink and orange on green eggs currently. The lighter the eggs, the lighter colors you can use. If I were going to do it with a dark egg laying hen like BCM, I would use a blue or purple, though I haven't tried that yet.