There’s a widespread claim in modern heraldic circles that furs and proper charges are neutral for contrast purposes (eg Wikipedia on the Rule of Tincture), and many of them cite this passage from Fox-Davies:
Furs may be placed upon either metal or colour, as may also any charge which is termed proper.AC Fox-Davies, A Complete Guide to Heraldry (1909), p 86.
With all due respect to Fox-Davies, generally considered the leading light of Victorian-era English heraldry, I think this is mistaken.
Vair (and its cousins like potent), being equal parts color and metal, are neutral for the same reason that checky fields are — but ermine and its variously-colored equivalents are predominantly either light or dark, and it makes sense that they should be considered as such for the purposes of contrast.
Earlier sources seem to agree that for the purpose of the rule of tincture, ermine should be treated as a metal. One of the earliest printed heraldic treatises in English is by Legh, who writes:
You must understand, that Ermine is no colour of him selfe: but a compound with a mettal, and serveth as mettal onely, without breaking of any Rule and is speciall good armes, both of it selfe, and with other.Gerard Legh, The Accedence of Armorie (1563), p. 32.
This is echoed in Guillim (A Display of Heraldry (1679), chapter III, section I, page 14), although he amusingly distinguishes the furs from the metals in depictions of mantling, on the grounds that the furs are warm while the metals are cold, and thus would not serve well to repel cold weather.
To return to Fox-Davies, in the same paragraph as the first quote above he writes:
It is also correct to place ermine upon argent. But such coats are not very frequently found, and it is usual in designing a coat to endeavour to arrange that the fur shall be treated as metal or colour according to what may be its background. The reason for this is obvious. It is correct, though unusual, for a charge which is blazoned proper, and yet depicted in a recognised heraldic colour, to be placed upon colour; and where such cases occur, care should be taken that the charges are blazoned proper.
Indeed, such designs are not just unusual, but vanishingly rare — we find plenty of cases of ermine upon colors, and counter-ermine on metals, but I’ve looked at an awful lot of pre-1600 armory and I can’t think of a single example of light furs on metals, or dark furs on colors, or vice-versa.
(And yes, it wouldn’t surprise me if you could find one or two historical examples of ermine charges on an Or field, just as we can find other examples of low contrast charges… but we understand those to be exceptions, or specific regional variations such as the special status of gules for contrast in central Europe, or the pattern of vert trimounts on color fields in eastern Europe, rather than standard practice.)
To say that it’s “correct” but simply never done seems calculated to confuse, and leads too many modern practitioners astray. I suspect this is a case of Fox-Davies getting swept up in some Victorian systematizing effort, despite the evidence that’s staring him in the face.
Rather than reading Fox-Davies as declaring furs and proper charges immune to the rule of tincture, it’s much more sensible to agree with his advice that “fur shall be treated as metal or colour according to what may be its background” and to thus return these tinctures to the same rules as we use everywhere else.
Conveniently, this is precisely what the College of Arms of the SCA has done, and I believe it places us in alignment with historical practice, even if some modern reference works claim otherwise.