Given that the red side is opposite to black and the red side is face down, the black side must be face up. The brown side is adjacent to blue, and blue is adjacent to white. This means brown, blue, and white are in a sequence around the cuboid. Since red is down and black is up, the remaining four sides must accommodate brown, blue, and white. If brown is adjacent to blue, and blue is adjacent to white, then brown cannot be opposite to blue or white. Therefore, brown must be opposite to the remaining colour, which is white.