Royal Icing

Is the lemon juice just to add flavor or does it do something else?

I used this recipe to make some collars (my first time making these) for a football-themed cake. The icing was great! I let the decorations dry/set up for 2 full days and they looked perfect! The decorations were red, white, blue. Next time I'm probably going to allow more time and let the darker colors dry a bit longer before adding the white, as some of the red & blue bled into the white. Also, when I put them on the cake, some of the green from the "grass" (buttercream frosting tinted dark green) bled through the white. Does anyone have any suggestions on a "sealant" which I could, perhaps, paint on the back side of the collars to keep darker colors from bleeding through? I was thinking maybe painting on edible glycerine, or perhaps just piping some extra white buttercream on the backside of collars before laying them on the cake...? Many thanks in advance :)