Well they make "Tan" RIT dye, that is very close to the color of those pants. I would guess, if you dyed the grosgrain, with that, it would make the white part tan, and the brown parts, a darker, richer, brown. (though you certainly should try it on a small piece first. )
And thanks for posting the info, on where to get some, I do believe I'll pick some up myself.![]()