There is absolutely no concerns today in modern
hot composting processes when using any kind of newspapers. All
newspapers today use totally 100% biodegradable inks. mostly soy based
inks. Even some heavy metals in papers, is no big deal for composting
microbes. (NOTE: There are heavy metals in healthy carnivorous manures
like chicken poop!) Only the old organic books used to discourage
newspapers in composting. Also it is safe to use more shredded,
moistirized, cardboards in a hot pile, if you run out of browns. I only
use paper or 100% cotton rags in my piles, when I'm really low on
leaves or straw.
The main ingredients in paper products that aerobic microbes love
is of course the carbon. Many papers have decomposed cellulose in it.
The carbons, carbohydrates, starches, fats, and cellulose from "browns"
(high carbon sources) are all important in the manufacturing of humus
in the soil. The "greens" don't do this. The "greens" (high nitrogen
sources) normally supply the protein/ammonium nitrogen, aerobic
microbes, most plant nutrients, and the internal heating and
reproduction foods as the microbes digest, grow, and multiply in the