I'm pretty sure his butt floss was white in ESB. And I think it's the same for Jedi. Heck, it might even be the same stuff used on the knee pad, which was white.
However, there is a knee strap floating around (AOSW), which is a PrePro knee, that has the strap painted yellow from over-spray.
As far as the white straps looking grey or misted, I really think they are just old and dirty. Although, I wouldn't completely rule out they dirtied the elastic up a bit.