Congratulations on your purchase! I don't own a Pedersoli Tryon, but by all accounts they are outstanding rifles.
Regarding your question, I can tell you from personal experience that Bore Butter turns brown in time. A lot of people use it because of the advertising hype, and it works as a patch lube, but it will oxidize in a surprisingly short time and become the same color as rust.
If your bore is cleaning up well, that may be the situation with your rifle. I would suggest dismounting the barrel from the stock, and remove the nipple. Put the breech end in a small bucket with a funnel in the muzzle and pour really hot water through it until the water looks clear. Some Dawn dishwashing liquid will help dissolve the grease. Then put in a rod with a jag and patch and pump the hot water up and down the bore. You probably have Bore Butter in the patent breech and this will help flush it out. When you are satisfied the the bore and breech are clean, flush with clear hot water (without detergent), dry the barrel inside and out, and stand it muzzle down in a safe place to drain any water that may be left in the breech. The heat will help dry the water. Dry patch the bore again, and treat it with a surface protectant, some sort of grease or oil that won't evaporate. Wipe this out before loading and shooting.
We will probably get some comments about "flash rust" from the hot water treatment. This has been discussed many times. All I can say is that hot, soapy water works well for me. My hypothesis is that minerals or chemicals in the water contribute to the flash rust phenomenon. I use distilled water, which costs 89 cents per gallon at the local supermarket and find no problem with flash rust. A gallon will be plenty, more than enough to clean your barrel.
Good luck with it! Let us know how it turns out, and by all means submit some photos. We love gun pictures.
Best regards,
Notchy Bob