When i wipe the skin between my vulva and my thigh the toilet paper turns brown? as if its dirty in the crease where my vulva meets my leg. whats this

Oil buildup?: Without seeing it (and don't send me pics :)) it sounds like sebum or oil buildup. The medical term is "terra firme-forme" rubbing it gently with isopropyl alcohol should remove it. If the area becomes irritated try vaseline. However with all that said, you should have it looked at by a dermatologist but mention the above term when going. Hope this helps! good luck!