Hi thanks for your question.
The knot you are mentioning after applying tourniquet is usually due to accumulation of blood in muscle and skin below the tourniquet. This disappears once tourniquet is removed. The cause for turning this swelling into sore most probably could be a small bruise
produced due to improper placement of the tourniquet or it could be a small haematoma
due to rupture of some superficial vein
again due to improper placement of tourniquet. The color of knot what you are mentioning occur during the normal process of healing of bruise or resolution of haematoma. Keep it clean. You can apply alcohol swabs. Nothing to worry it will get resolved soon. Consultation with operating surgeon is advisable
Hope this answers your question.