From the AKC standard:
"Color
Always black with rust to mahogany markings. The demarcation between black and rust is to be clearly defined. The markings should be located as follows: a spot over each eye; on cheeks; as a strip around each side of
muzzle, but not on the bridge of the nose; on throat; triangular mark on both sides of prosternum;on forelegs from carpus downward to the toes; on inside of rear legs showing down the front of the stifle and broadening
out to front of rear legs from hock to toes, but not completely eliminating black from rear of pasterns; under tail; black penciling on toes. The undercoat is gray, tan, or black. Quantity and location of rust markings is important and should not exceed ten percent of body color.
Serious Faults--Straw-colored, excessive, insufficient or sooty markings; rust
marking other than described above; white marking any place on dog (a few rust or white hairs do not constitute a marking).
Disqualifications--Any base color other than black; absence of all markings."
http://www.akc.org/breeds/recbreeds/rotty.cfm