It is a typo and the words soufre roux are to be understood in their literal meaning.
Theses line are from Les campagnes hallucinées, this collection of poems is the first book of his "social trilogy" (Les Campagnes hallucinées, Les Villes tentaculaires and Les Aubes) in which Verhaeren describes the transformation of the rural landscape brought about by the pollution caused by industrialisation at the end of the 19th century. In the three parts of his trilogy Verhaeren uses the word soufre several times¹, only in Les campagnes hallucinées is it spelt wrongly. Some later editions corrected the error. Whether the mistake was in the original manuscript or due to a misprint I don't know, in some later editions of Les campagnes hallucinées the spelling is rectified.
In all the occurrences soufre/souffre is referring to the chemical element. There's absolutely no way Verhaeren would mention anything like the Philosopher's stone. Verhaeren describes the destruction of nature by smoke, soot and sulphuric acid deposits. In these lines the souffre roux is one of the components of the pollution that reflects on the feathers of the crows. Colours, light, shadows, all play an important part in this work and contribute to the general atmosphere. Whether the sulphuric dust is actually red is not relevant, sulphur is part of the pollution and it is mixed with other polluting components.
As an art critic Émile Verhaeren promoted impressionism. And it's no wonder that the landscapes he describes in his trilogy brings to my mind some works by impressionist painters such as Monet, Pissaro, Guillaumin...
¹ Un air de soufre et de naphte s'exhale,
Un soleil trouble et monstrueux s'étale; (Les villes tentaculaires)
Les cieux vivants sont dévorés par les fumées ;
L’herbe saine, la plante vierge et les moissons
Mangent du soufre et des poisons (Les aubes)