Without looking I recall 68 is burnished saddle and tr189 is the same color interior. F is the vinyl top color and I cannot remember the name but it is a shade darker in a brown color.
Yes --I owned a 70 GS 455 convert 4 speed with colors like that. It was your typical t*** brown car of this era.
F code is just called Dark Brown in Buick sales and service literature. Same as mine. Brown, Brown, Brown all around.