I think the black sporran looks fine. It has an understated look, which I think is often good for business attire, particularly when you are trying to make a good impression. You can add a bit more flash, if you like, the next time you wear a kilt to work. I like all three options, though the brown sporran has a slightly rough or very casual look.

I also would not have noticed that you were wearing a casual style kilt, if you had not mentioned it, I guess because of the vest /waistcoat. Go for it!!