  Based on a grid of hydrostatic spherical COMARCS models for cool stars we
have calculated observable properties of these objects, which will be mainly
used in combination with stellar evolution tracks and population synthesis
tools. The high resolution opacity sampling and low resolution convolved
spectra as well as bolometric corrections for a large number of filter systems
are made electronically available. We exploit those data to study the effect of
mass, C/O ratio and nitrogen abundance on the photometry of K and M giants.
Depending on effective temperature, surface gravity and the chosen wavelength
ranges variations of the investigated parameters cause very weak to moderate
and, in the case of C/O values close to one, even strong shifts of the colours.
For the usage with stellar evolution calculations they will be treated as
correction factors applied to the results of an interpolation in the main
quantities. When we compare the synthetic photometry to observed relations and
to data from the Galactic Bulge, we find in general a good agreement.
Deviations appear for the coolest giants showing pulsations, mass loss and dust
shells, which cannot be described by hydrostatic models.
