I saw a performance of Macbeth last summer (at the University of Florida in Gainesville) where the director chose a minimalist staging in the extreme. The characters were dressed in loincloths and carried spears (picture a combination of The Flintstones and stereotypical cave man dress). The cast remained barefoot for most of the performance. Lady Macbeth wore a skimpy, brown loincloth/dress (again, picture Wilma Flintstone); during the scene where she convinced Macbeth to kill Duncan, Lady Macbeth appeared topless as she convincingly seduced her husband. The staging worked for the most part, and it was certainly one of the most unusual Shakespearean productions I have attended.
That's a terrific question. The answer, however, is problematic. If you are putting on the play and it is true to its time period, then you would need to research how women of stature and royalty were dressed nearly a thousand years ago in Scotland. Good luck with that. If you are staging the play in modern times, things will be much easier as you have a familiar frame of reference.
But whatever time period you choose, remember that Macbeth is a play filled with opposites and contradictions... fair is foul and foul is fair. Thus, however you dress Lady Macbeth, make it lovely and feminine, for she is really treacherous and masculine. Early in the play, go with light colors and frills, clothes that are at once enticing and tasteful, simple, yet elegant. And when she has become the queen, definitely bedeck her with jewelry: bangle earrings and jeweled necklaces, gold bracelets and sparkling crowns.