Before I added elastic to my knees and buttplate, I stretched the pieces out onto a board with thumb tacks. I spray-painted the stretched elastic a light-grey color.
The pull-out poster in the BF magazine (Star Wars Galaxy Collector 1998) shows the knee straps to be a greyish color. The model wasn't wearing the butt-floss in that picture.
I think the idea is to make it a color that blends with your jumpsuit; as someone stated above, you want the focus on the overall costume, not your thong.