Montblanc's refill range runs to black and blue in F, M and B, and to red and green in M only (as of 2008). A brown ballpoint refill is not among them and was not in the range earlier either; there is thus no factory answer for that colour.
Anyone who still wants to write in brown must fall back on refills from other makers. Bear in mind that Montblanc uses a refill of its own shape and dimensions, so not every refill sold over the counter will go into the twist mechanism without modification. Since the range changes over the years, it is also worth checking the current choice of colours and widths with a dealer or at a boutique before ordering.
The refill has no bearing on collector value. It is a consumable, and a refill that is not original does nothing to diminish the worth of a Meisterstück.