Is Sulky Cotton colorfast?

Yes, all of the Sulky threads are colorfast in both hot and cold water. HOWEVER, there are laundering cautions regarding not using bleach or detergents containing optical brighteners with any natural fiber thread such as Cotton or Rayon. Polyester thread like Sulky Poly Deco can be bleached and abused much more than the natural fiber threads. See the Q & A section on Laundering