What is non-copying ink? The rule and its comment do not say.
The internet is not particularly helpful on direct inquiry, but we do find a definition for the forbidden "copying ink":
Copying ink, a peculiar ink used for writings of which copies by impression are to be taken.Merriam-Webster says "ink suitable for writing or typing that is to be copied by direct transfer (as in a copying press)."
Wikipedia has a good article on "duplicating machines" that discusses copying presses as well as another method implicitly forbidden by Rule 32(a), the "spirit duplicator" or, as we called them back in elementary school, "ditto machines." The ink used in those was almost invariably purple. (I can still remember the smell, which delighted us schoolchildren for some reason.) Turns out that the same aniline purple is used in blue jeans.
Anyway, so now you know that you haven't been breaking Rule 32(a) all along. Probably.
[UPDATE: Jim refers us to the 1902 Encyclopedia Britannica:
Copying Ink.—Ink which yields by means of pressure an impression, on a sheet of damped tissue paper, of characters written in it is called copying ink. Any ink soluble in water, or which retains a certain degree of solubility, may be used as copying ink. Runge's chrome ink, being a soluble compound, is, therefore, so available ; and the other logwood inks as well as the ordinary ferrous gallate inks contain also soluble constituents, and indeed are essentially soluble till they are oxidized in and on the paper after exposure to the air. To render these available as copying inks it is only necessary to add to them a substance which will retard the oxidizing effect of the air for some time. For this purpose the bodies most serviceable are gum arabic or Senegal, with glycerin, dextrin, or sugar, which last, however, has the disadvantage of rendering the ink sticky. These substances act by forming a kind of glaze or varnish over the surface of the ink which excludes the air. At the same time when the damp sheet of tissue paper is applied to the writing they dissolve and allow a portion of the yet soluble ink to be absorbed by the moistened tissue. As copying ink has to yield two or more impressions, it is necessary that it should he made stronger, i.e., that it should contain more pigment or body than common ink. It, therefore, is prepared with from 30 to 40 per cent, less of water than non-copying kinds ; but otherwise, except in the presence of the ingredients above alluded to, the inks are quite the same.The solubility is evidently the problem; Jim notes that Mississippi's humidity would've made these inks particularly unsuitable for a pre-A/C court, and they don't seem archivally suitable in any event?]
... The rule also says "on white paper without the name of any person or advertising matters on the paper." Does that forbid watermarks on bond paper? We've never had a brief rejected on that basis .... The "original," I'm pretty sure, sits in a file while the justices read the copies. And here recently, I've switched to 28-lb copy paper instead of bond, which does a terrible job of holding laser toner.
... One more tidbit: I've seen some briefs name both the MSSC and the COA on their covers, apparently recognizing that the filer doesn't know which court will hear the appeal. Don't do that. "Prior to notification by the clerk that the case has been assigned to the Court of Appeals, all pleadings shall be captioned in the name of the Supreme Court." M.R.A.P. 32(b).