Most herbs are going to discolor. Lye monster!!
Calendula petals will not and look lovely in soap. Most soapers, myself included, add any additives at thin trace. If using oatmeal, some soapers like to add it to the oils before the lye to avoid clumping.
You can add anything you want, but add at a rate of 1T per pound to start. Some stuff is too scratchy and shouldn't be added (unless it's just to the tops after it's poured into the mold - that way it can be sliced off). Oatmeal should be ground fine or use colloidal or baby oatmeal. Peppermint leaves turn brownish, but are a nice additive. I hate lavender buds in soap as they look like mouse turds. Adding these to the tops can be aesthetically pleasing. Chamomile can be added, but should be finely ground. Seaweed is nice - and it pretty much stays that dark dark green.
I've added sterilized sand before for a scrubby beach soap. Pumice comes in all kinds of colors and makes a great foot or hand soap. Cornmeal can be added for a gardeners bar. I've also added ground orange and lemon zest. It doesn't turn brown, but doesn't have much color either.
