What is the difference between "sombre" and "foncé" ?
sombre can be used to describe the overall appearance of colour whereas foncé is generally used in unison with another adjective such as bleu foncé → "dark blue".
What is the difference between soulier and chaussure?
soulier is is a somewhat old-fashioned term but it is still widely used in the French fashion industry. Often translated as footwear, it includes things like slippers and sandals, which aren't quite the same as chaussures.
Why des? I thought its "de" if there is a plural with a plural adjective?
That's only true if the adjective immediately follows the "des." If it were "big dark shoes" it could be "de grandes chaussures foncées."