Many different formulas have been used for Viton over the years, but black and brown Vitons are functionally interchangeable. Depending upon the formulation, the one may be slightly softer than the other, and black ones contain carbon black. The 'Chromassure' uniform color coding system was adopted to simplify identification of the different polymers, but it obviously isn't being followed as fluorocarbon o-rings (which include Viton, Kalrez and Kel-F) are all supposed to be black.
Viton is a duPont trademark for a specific fluoroelastomer polymer (typically abbreviated as FPM) made with vinylidene fluoride-hexafluoropropylene copolymer - the so-called Viton A resin. Magnesium oxide (MgO) is added as a proton (acid) acceptor to react with the small amount of HF that results from polymerization of the resin. MgO is brown, thus an excess of MgO causes the finished product to be brown unless sufficient carbon black is added as a colorant - AHAH!! By weight, the typical (black) Viton is approx 11% MgO, 18% C and 1% Diak No. 1 (a curing agent). The balance is Viton A resin. Different manufacturers use slightly different formulas, so their o-rings are slightly different in color and physical properties. (See, 'Practical selection of elastomer materials for vacuum systems', Norman Peacock; J. Vac. Sci. Technol., 17(1), Jan/Feb 1980, p. 330-336, which discusses polymer chemistry, seal design, seal mechanics and compression set, outgassing and permeation, and radiation damage of polymers).
When Viton (the cured product) is exposed to acid (eg. HCl or HF vapors) it will produce minute quantities of water vapor. This is not a problem in most situations, but in ultra high vacuum or air sensitive compound handling systems it can be! Remember, there is an excess of MgO added when the polymer is compounded, and the reaction that produces the water is MgO + 2 HX —> MgX2 + H2O (where X is the acid anion - such as F- or Cl-, but can also be a polyprotic acid anion such as HSO4-, H2PO4-, etc).
In a quarter century of experience I have yet to hear a researcher complain about either the black or the brown Viton, or who has expressed a preference for the one over the other. I have however noted that some of the brown ones have a slightly rough or matte surface finish which I don't recall seeing on the black ones - they are always glossy and smooth. I have also never had a complaint about leakage that I could attribute to the matte brown ones, and have come to the conclusion that Viton is Viton is Viton.
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