  In the last decade, white-light illuminated Fabry-P\'erot interferometers
wave been established as a widely used, relatively simple, reliable, and
cost-effective way to precisely calibrate high-resolution echelle
spectrographs. However, Terrien et al. (2021) recently reported a chromatic
drift of the Fabry-P\'erot interferometer installed at the Habitable-zone
Planet Finder spectrograph. In particular, they found that the variation of the
etalon effective gap size is not achromatic as usually assumed but in fact
depends on wavelength. Here, we present a similar study of the Espresso
Fabry-P\'erot interferometer. Using daily calibrations spanning a period of
over 2.5 years, we also find clear evidence for a chromatic drift with an
amplitude of a few cm/s per day that has a characteristic, quasi-oscillatory
dependence on wavelength. We conclude that this effect is probably caused by an
aging of the dielectric mirror coatings and expect that similar chromatic
drifts might affect all Fabry-P\'erot interferometers used for calibration of
astronomical spectrographs. However, we also demonstrate that the chromatic
drift can be measured and in principle corrected using only standard
calibrations based on hollow cathode lamp spectra.
