In two identical containers with similar amounts of pesto from the same batch I covered one container with a layer of olive oil and in the second container put enough lemon juice to coat the top of the pesto.
Here are the containers ready to go in the fridge. The container with the layer of olive oil always being on the left. The container with the lemon juice looks brighter from the get go, but the oil gives it the darker appearance.
24 hours later: Both have had a little bit of browning at the top.
48 hours later: Not much change since 24 hours.
I decided to stop the experiment 48 hours in because I wanted to use some of the pesto. Essentially both methods worked about the same. The container with the oil looked darker because of the color of the oil but the actual pesto was about the same. The top layer of both containers darkened slightly but it was an extremely thin amount that you could either skim off or mix in. I still prefer to use the lemon juice because it doesn't add extra fat and brightens the flavor of the pesto. Pesto should really only be kept for a couple of days in the fridge beyond that I would suggest freezing it.