plgcolbg:  Returns the background color
(cmap0[0]) by 8-bit RGB value 

DESCRIPTION:

    Returns the background color (cmap0[0]) by 8-bit RGB value. 

SYNOPSIS:

plgcolbg(r, g, b)

ARGUMENTS:

    r (PLINT *, output) :    Pointer to an unsigned 8-bit integer (0-255)
    representing the degree of red in the color. 

    g (PLINT *, output) :    Pointer to an unsigned 8-bit integer (0-255)
    representing the degree of green in the color. 

    b (PLINT *, output) :    Pointer to an unsigned 8-bit integer (0-255)
    representing the degree of blue in the color.