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