My plan is to make a faux wedding cake for my wedding and have a cupcake decorating station. I made a royal icing and poured it over the styrofoam cake dummies and I let it sit for a week because it still seemed sticky on top. I took a look at them today and they've started to get some yellow stains all around! I've read so many people's posts saying they used royal icing and it worked so well for them. No one has mentioned any yellowing so what am I doing wrong? I don't want to try spackle because I feel it won't be smooth enough for my ocd! Any suggestions? 

Did you add any kind of citrus? I normally add a bit of lemon juice or cream of tartar. I can't remember the specific reason behind it but it has something to do with the egg yolks. My chef always told me to have some sort of acid.