I am attempting to make a Booger cake for my sister (Booger is her nickname) but she wants it to be a red velvet cake with cream cheese icing. My big questions are 1) can i color the Cream Cheese icing green? 2) is it better/possible to do a layer of cream cheese under a green layer of royal icing? 3) any better ideas about getting a great slimy green top layer but still have the cream cheese underneath? I know that Wilton has the spray cans, but i've never used them. i'm really wanting more of a gel finish, but this will probably be a decently sized cake and the gels i've seen are tiny. I'd appreciate any help... BTW, her bday party is on SUNDAY!

Yes, you can color cream cheese icing green.
You can also purchase large cans of untinted piping gel and tint the gel with icing colors.
Royal icing dries rock-hard, not a good idea for icing a cake.