Monochromatic X-rays have been proposed for medical imaging, especially in the mammographic energy range. Our previous investigations have shown that the contrast of objects such as lesions or contrast media can be enhanced considerably by using monochromatic X-rays instead of the common polychromatic spectra. Admittedly, only one specific polychromatic spectrum and one monochromatic energy have been compared so far. In this work, we investigated the contrast yielded by a series of different X-ray spectra obtained by varying tube voltage and beam filtering. This resulted in spectra of different mean energies and spectral widths. The objects under examination were aqueous solutions containing different chemical elements such as I, Gd, Dy, Yb, and Bi. A monoenergetic spectrum at 17.5 keV was obtained using a mammographic X-ray tube with a Mo anode and a monochromator equipped with a HOPG crystal. Moreover, we simulated quasi-monoenergetic spectra at different energies and with different widths. As a result, we demonstrated that in many cases spectra with an energetic width of some keV yield an equivalent contrast to monoenergetic radiation at the same energy. Therefore, the advantage in image contrast of monochromatic X-rays at 17.5 keV over narrow-band polychromatic X-ray spectra obtained by appropriate filtering is only slight. Thus, the additional expenditure on a mammography system with HOPG monochromator that can deliver only a small X-ray dose and the unfavorable slot-scan geometry can be avoided. Moreover, we carried out simulations of monochromatic versus polychromatic spectra throughout the whole radiographic energy range. We found advantages in using monochromatic X-rays at higher energies and thicker objects that will justify their application for diagnostic imaging in a number of specific cases.