I've heard of quilters giving their quilts a "tea bath" to tone down the patterns on the fabrics. Sort of giving the quilt and aged look. You think you could do that with a tartan? Maybe to make it look more like a hunting or an ancient? Apparently you use actually use tea or sometimes even instant coffee as the dye. Hence the name.
I'm guessing any poly/viscose wouldn't hold the color at all since it's synthetic. It's prolly work on wool though....?? *pondering*