That brown is typical of nitrogen compounds, so whatever is burning had nitrates or similar in it.

Ah - another post identified nitric acid. Nitric acid doesn't burn, but as soon as it would spill and attack anything organic nearby, that would burn to brown smoke if ignited after it got converted. For instance, the nitric would turn dry grass to nitrocellulose (guncotton). Although it needs sulfuric acid too for best results.

That brown is typical of nitrogen compounds, so whatever is burning had nitrates or similar in it.

Ah - another post identified nitric acid. Nitric acid doesn't burn, but as soon as it would spill and attack anything organic nearby, that would burn to brown smoke if ignited after it got converted. For instance, the nitric would turn dry grass to nitrocellulose (guncotton).