@Clifford MakeWS2811(...) does take a pointer, but it looks like it makes a copy of the options in C.
However the returned instance has a Leds(nChannel) method, giving direct access to set the colours of LEDs. You can reduce the colour value to reduce the brightness (approximately).
... opt.Channels.Brightness = ... original value 0 to 255 ws, err := MakeWS2811(&opts) ... ws.Leds(0) = 0xff //bright red ws.Leds(0) = 0x7f //half as bright ws.Render()
Note each time the C lib renders, it still scales the colours you set by the original brightness.