I would ignore the "rule" about what color sporran to wear according to the time of day. I would buy a sporran that you like and that will look good with your kilt. A two tone sporran works for me. I personally like the combination of brown and black leather together and currently have two pairs of shoes that are black with brown trim, and that's what I specified for the sporran that I recently ordered. If you are going to wear a kilt as part of a costume, the "rule" might matter, but if a kilt is going to be part of your 21st century wardrobe, you are allowed to do what YOU want and wear what you think looks good. If your sporran doesn't match your shoes, the earth will not stop rotating, and if anyone has a problem with it, let it be their problem!