One of the really neat things about the X Marks tartan is that the yellow line is actually woven with two different shades of yellow threads, one bright and one more muted. So whether you go for the yellow or the mustard ties, either will coordinate well with the yellow of the tartan.

If you are looking for something eye-catching and flashy, go for the bright yellow. If you are after a more subtle or toned-down look, go for mustard. Either will look just fine.