White cotton thread was used for the quilting when making the super trooper, when the gloves were dyed grey for Empire the thread took on the color. They faded out due to poor storage between films, and that is why the stitches appear dirty white (that's also why the gloves appear lighter) in Jedi. Add another 30+ years of storage and travelling exhibitions and they have continued to fade over time, which is why in the MoM and AoSW photos the gloves are so light grey.