AnD4D Duh, I never even thought to look there.
I can confirm that this is a matter of inaccuracy. See the following test capx. I replaced the Tilled Background walls with single sprites and there are gaps between them.
@zenox98 You are right. There aren't any color options in expressions. Perhaps they are not implemented yet..?
@AnD4D Why don't you elaborate on how do you wish the plugin to be improved? Perhaps there is some workaround or Ashley could enhance it in future releases.