NAME
       SetRGB4 -- Set one color register for this viewport.

    SYNOPSIS
       SetRGB4(  vp, n,   r,    g,    b)
                 a0  d0  d1:4  d2:4  d3:4

        void SetRGB4( struct viewport *, short, ubyte, ubyte, ubyte );

    FUNCTION
        Change the color look up table so that this viewport displays
        the color (r,g,b) for pen number n.

    INPUTS
        vp - pointer to  viewport structure
       n - the color number (range from 0 to 31)
       r - red level (0-15)
       g - green level (0-15)
       b - blue level (0-15)

    RESULT
        If there is a colormap for this viewport, then the value will
        be stored in the colormap.
       The selected color register is changed to match your specs.
        If the color value is unused then nothing will happen.

    BUGS

        NOTE: Under V36 and up, it is not safe to call this function
        from an interrupt, due to semaphore protection of graphics
        copper lists.

    SEE ALSO
       loadrgb4() getrgb4() graphics/view.h