Forgive me if someone already answered this, but from what I understand, it did exactly what it was told to. Your original image was mostly grey and black, so the two colors it chose to downsize to were grey and black. It doesn't matter if you set it to "L" or "RGB", since you gave it a predominantly grey and black image. As the other comment mentioned, you can create a very small image where the desired black & white palette is encoded into a minimal number of pixels, and pass this to the quantize method.