Every time I bake lemon bars (regardless of recipe), I seem to end up with a foamy cake-like layer on top of the lemon curd. I'm not really sure where I am going wrong, but it is definitely a technique over recipe thing because regardless of the recipe I have this problem.   EDIT: I make my own lemon curd, I don't buy it. Despite me varying recipes, and trying to vary my technique I still end up with a foamy cakey layer on top. The lemon mixture on top of my lemon bars consists of around (depending on recipe): 4 eggs, 300g sugar, juice of 2-3 lemons, zest and 55g flour.  
I strongly suspect that your food coloring may have had something to do with it if you were using liquid food coloring as these often have flavors you wouldn't expect. That plus the acidity from the coffee (yes, coffee is acidic) and the cream of tartar (powdered acid) could simulate an orange flavor.   Gel colorings are much better as they give much more color without flavors, I'd suggest those in the future, just remember a very little bit goes a long way.  