Your rainbow cake looks perfect! The Elmos are adorable, too! Great job, were the kids and mom thrilled? The vanilla mousseline looks perfect, too, when I saw the photo the first thing I thought was that you used the meringue, so it looks quite pale.
Did you end up subbing the gel food color for some of the milk, or just adding it?
When I make vanilla mousseline, I add more extract than Rose does, about 1 tsp per cup of buttercream if pairing it with chocolate, a little less with non-chocolate.
When you add the hot sugar syrup to the whites, they expand quite a bit and become more firm and sticky, especially as they cool. I guess that’s sort of marshmallow-y.
Now that I’m back on this thread, I see that I missed a question earlier- yes, the Italian meringue is just the egg white portion of the recipe. There’s a stand-alone recipe for it in the Cake Bible section on meringues, p.298. In RHC, the lemon meringue cake is an example of a cake frosted with Italian meringue.