Does cancer usually show up as white or black on an x-ray?

Grey-to-White. When looking at an x-ray, air is black and bone is white and everything else is somewhere in between the two. In general, cancers will look white but so will many, many other things. When looking for a cancer, we are looking at the shape, size, and relationship with adjacent structures. All things considered, it is a lot safer to read a radiologist's report than try and interpret a film.
Either. On an x-ray, white means dense tissue (bone, teeth, etc), while dark means tissue with high water or air content (like the air-filled stomach or lungs). So some cancers are white on x-ray, while others are black.