royal icing is your best bet.
you should also use a craft spray protector on top of it after it dries... i cant think of the name of it right now...but its available at micheals or any craft store. but it helps the cake last a litle longer... because the cake will yellow being in a window.
if you're going to use a coloring that includes red (purple, orange, pink, etc) make sure you invest in some fade proof stuff or your purple will fade to blue, your orange will fade to yellow and your red will...well, just fade away, potentially within a matter of days.
I use Chefmaster brand and it works great!
You can cover them in all crisco buttercream, fondant, royal icing (white will yellow after time), Perma Ice (I think that is the right spelling) or spackle (for walls).
I believe the Perma Ice and I know the spackle can be colored with craft paints, so they colors shouldn't fade over time. I wouldn't use those with my regular cake supplies though.
I did a fake cake for the Fair last year and i did regular Butter cream i even added the vanilla flavoring so it would be realistic and it work great just be careful when transporting it cause it will flake a little
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