  Using Monte Carlo simulations of globular clusters we developed a method
separating metallicity effects from age effects on observed integrated ugriz
colors. We demonstrate that these colors do not evolve with time significantly
after an age of 4 Gyr and use Bayesian statistics to calculate a probability
distribution function of the metallicity. We tested the method using the M31
globular cluster system and then applied to explain the observed color
bimodality in globular cluster sets and tidal effects on it. We show that the
color bimodality is an effect of a nonlinearity in the color-metallicity
relation caused by stellar dynamics on the Giant Branch, that colors including
only the UV show a weaker bimodality than those subtracting from visual bands
and that cluster sets with a distinct bimodality are in principle older than
those with only a weak bimodal distribution. Furthermore a bimodal color
distribution of coeval clusters implies a bimodal metallicity distribution, but
a unimodal color distribution does not imply a unimodal metallicity
distribution. The tidal field can finally shift the modes of the color
distribution and therefore cause a bimodal color distribution. This work
presents results obtained between 2011 and 2012 in the Astronomisches
Rechen-Institut, Zentrum f\"{u}r Astronomie der Universit\"{a}t Heidelberg,
M\"{o}nchhofstra{\ss}e 12-14, 69120 Heidelberg, Germany.
