The Sign of The Four - Arthur Conan Doyle

i have some english homework,
and we have to analyse an extract and answer the following question:
"How does Conan Doyle use the setting to create atmosphere?"

there's a quote in the extract:
"Mud-coloured clouds drooped sadly over the muddy streets."

i have an idea to analyse the quote and a point in the back of my head but i can't put it into a sentence. colours are commonly used to create meaning within the novel but how could you interpret "mud-coloured clouds"?