Why is my homemade soap always coming out brown?

I'm trying to perfect some of my professor's recipes for a gen chem lab. My ingredients between all the batches are canola oil, veggie oil, olive oil, lard, shortening, and then of course DI H2O and NaOH. Most are coming out brown (they still lather like normal) but my professor has never seen it before and he wouldn't like it in the lab. The only lead I have is the recipes with shortening have been creamish colored, but any recipe with lard is brown...ideas?