I usually do a layer on top that is unscented or scented with something complementary that won't turn brown. I prefer to do the top third that way, but I have also done half and half. That way I can have some color besides brown. I have done slab swirls, but no matter how 'fat' I make the unscented/colored swirls, it doesn't take long for the brown to creep in and devour the swirl. So the unscented layer works fine for an unspecified amount of time. Recently I found a bar of vaniglia madagascar that I had made with the top third unscented, colored with grape mica and frosted with gold mica. It's about 9 months old, still smells fabulous, but I noticed that the brown has actually started to seep into the lavender color...it's light but noticeable.
I remember in one of my first soaping experiences I used SW Night Spirit and colored it with blue ultramarine. It was gorgeous and smelled fantastic. Within a week it had turned a yucky brownish blue that was completely unappealing>I was heartbroken, as I was so proud of that soap!!
I gotta say though, it may bother US to make a 'plain' brown soap, but people love the scents! I usually do logs so I can frost the tops. To me the brown is just so-so, but no one seems to care...for them it's unusual to have a brown soap in their shower. And if they say, what? vanilla? brown? I remind them of the rich color of vanilla extract and they are okay with it.