The barrels were painted a light color on the underside so they would hopefully look like a shorter 75mm barrel instead of the deadly 17 pdr. I think there was only one 17 pdr Firefly per tank troop so that tank was a key target for German gunners.
I've never done a Firefly before, but if I were to try it, I would paint the barrel light gray first. Then I would cover the underside of the barrel with something like Silly Putty for a mask. Then I would paint the rest of the tank in the normal camouflage scheme. When done, the gray would be uncovered by removing the Silly Putty.
I don't know if it was factory applied or done in the field.