Kshar Sutra thread process. Clarify?

Hi, I was getting treated by my doctor earlier in my home town one month ago for fistula in ano with kshar sutra, but I have got to transfer to a different city because of work, So my doctor referred to another doctor in banglore. Previously with the old doctor I have observed the kshar Sutra thread was yellow in colour after threading process I used to see yellow colour allover my bandage and it used be more painful, with new doctor I observed they were laying black colour thread and after threading process on the end of day I am seeing black colour over the bandage.it is comparatively less painful, What could be the reason for the change in kshar sutra thread Colour and pain associated with it could you please clarify? Thanks