I use a swiss meringue buttercream for all of my cakes but I am running into a problem and would like to know why it happens. When I ice my cakes, I put them into the freezer to set up for a few minutes, pull them out and smooth with a knife dipped in hot water and wiped off. I have notice that the colored icing kind of gets mottled a bit, but it has never been a big deal. The other night I had a beautiful white cake and when I started smoothing, the icing started changing to an awful yellow color.
What causes this, and what are the solutions? I feel like I am missing something obvious, but I can't figure out what. Any help would be appreciated!