WordWeb Online

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

See also: dirty, ill, sick, tasteless