WordWeb Online

1. In violation of good taste even verging on the indecent
   "an off-colour joke"
   - indelicate, off-color [US]
    
2. Humorously vulgar
   "off-colour jokes"
   - bawdy, off-color [US], ribald
    
3. [Brit] Somewhat ill or prone to illness
   - ailing, indisposed, peaked, poorly, sickly, unwell, under the weather, seedy, out of sorts, crook [Austral, NZ]

See also: dirty, ill, sick, tasteless