The Colour Liver
The Colour Liver
Thought to be due to the Whippet crosses used years ago. The Blue also is thought to be due to this cross, the Black and Tan due to the Manchester Terrier 
. Incidently the colour black was never in the original Bulldog so our black staffies are thought to have other terrier influence.
Interesting 
The Black and Tan 
The terrier used in cross with bulldog to create SBT was the (Old) English Terrier. "Some time about 1800/1820, the Bulldog was crossed with the Old English Terrier and produced the Staffordshire" (Beilby H N 1943. The Staffordshire Bull Terrier, page 7).
The breeds like White English Terrier and Manchester Terrier didn't yet exist. The coloration varied in the (Old) English Terrier. Most common was black&tan but there were also white, pied and even blue. (Brown T 1829. Biographical Sketches and Authentic Anecdotes of Dogs, page 342; Jardine W, Smith C H 1840. The Natural History of Dogs, page 206; Walsh J H "Stonehenge" 1859. The Dog in Helath and Disesase, page 74; Beilby H N, pages 9-10.)
B&T was considered as the only "true" color for English Terrier (Walsh, p. 74). Maybe the other colors were a bit unwanted because they reminded from the Bulldog / Bull Terrier (yes Bulldog blood was added to many terriers to give them needed courage and higher threshold for pain). And maybe the Bull Terrier (SBT) people prefered the coloration of courageous Bull Dog and didn't like the B&T which was a wimpy terrier colour.