This seems to be an endless discussion: why is dark comb dark? As was mentioned, burr and brace comb is dark, not just brood comb. I think Hepburn states the case clearly:

Comb which is used for brood-rearing will become darker with age and almost black, and more brittle (Hepburn 1998) because of the accumulation of faecal material (Jay 1964), propolis and pollen (Free and Williams 1974). The darker colour wax may contain a collection of
undefined contaminants accumulated over time. The huge turnover and shifting rate of wax is also evident given the fact that brood cell cappings in nests containing old and darkened combs are nearly as dark as the combs themselves. In the white combs of newly established swarms, the cappings are only slightly discoloured.

More recently, Namdar et al. (2007) published GC and GC/MS analyses of light and dark coloured A. m. ligustica and A. m. syriaca combs (Fig. 16.3). They found that, as beeswax ages and darkens, its n-alkane composition changes. The amount of even numbered n-alkanes (C22–C32), is significantly higher in darker coloured beeswax compared to light beeswax. They attributed these differences, at least in part, to the accumulation of cuticular residues known to contain C23 to C32 odd and even numbered n-alkanes. They determined the presence of odd and even numbered n-alkanes, and showed that there was a clear predominance of the C27 alkane, with only very small amounts of even numbered n-alkanes in the range of C22–C32. Also, darker beeswax contains on average about 3 times more even numbered n-alkanes than lighter coloured beeswax.

see: Namdar D, Neumann R, Sladezski Y, Haddad N, Weiner S (2007) Alkane composition variations between darker and lighter colored comb beeswax. Apidologie 38:453–461

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