Sentence:  Cerro Colorado is composed of granitic rocks covered with a dark-colored desert varnish. The Sierra Cucapa are a lighter reddish-brown and lie in the background on the far side of Laguna Salada. They are also covered with a patina of desert varnish but not as <WSD>darkly</WSD> as the surface of Cerro Colorado. ... A minor weathering feature of the desert is a thin, shiny, reddish-brown to blackish coating called desert varnish that occurs on some desert rocks. 
Meaning of the word marked with <WSD> tags:  With a dark appearance.