The base thread is 6-stranded white DMC embroidery floss. Each skein is unique as it is hand dyed.
I use procion fiber reactive dye, which is a permanent and colorfast dye. I soak and rinse my thread twice and then wash with a commercial detergent. Most thread should be colorfast.
Please keep in mind that very dark colors may have residual dye left after washing. To check the colorfastness, please soak a small piece in cool water and your soap preference (I recommend either dawn or orvus ) before stitching if you're worried. I have no control over which soaps are used so colorfastness is not guaranteed.