Different energies of X-ray radiation mean different transparency of an object. There is an example in my FAQ using a Nautilus shell.
Instead of compressing images of different energies to a single image today I subtracted the 70 kV image of a Nautilus shell from the 40 kV image.
The central parts of the Nautilus shell are more dense and show a significant higher difference. The core of the shell gets shiny. This is how it looks like:
In positive X-ray representation you can compare the results. Left hand is the compressed image of 4 different energy levels, right hand the difference image.