Maybe a good way of explaining it would be like this: if someone magically had a blue spine, this technique would still show their spine as being white (unless the doctor already knew their spine was blue and adjusted the parameters accordingly)?
My point is, showing spine as white on X-ray (well, that's probably easy, but generalizing this to other tissues and their "normal"/"expected" color probably isn't) would be tremendously helpful in diagnosis... "peeking inside patients without cutting them up" - and I'm wondering, what's stopping us from doing just that?
We see in a range of wavelengths 0.4--0.7 um. X-rays have no intersection with our visible range. Therefore the best you can do is define some function that maps x-rays into our color range, but things look very different in x-ray land than they do in color land. For starters, most things would be transparent if you could see x-rays (hence x-ray imaging!).
http://www.columbia.edu/~vjd1/electromag_spectrum.htm