plscolbg: Set the background color
      by 8-bit RGB value
    
      
    
      Set the background color (color 0 in cmap0) by 8-bit RGB value (see
      the section called “Color Map0”).
    
- 
          r(PLINT
- 
            Unsigned 8-bit integer (0-255) representing
            the degree of red in the color.
           
- 
          g(PLINT
- 
            Unsigned 8-bit integer (0-255) representing
            the degree of green in the color.
           
- 
          b(PLINT
- 
            Unsigned 8-bit integer (0-255) representing
            the degree of blue in the color.
           
      Redacted form: plscolbg(r, g, b)
    
      This function is used in examples 15 and 31.