  It is shown that the inverse of the ghost form factor in the Hamilton
approach to Yang-Mills theory in Coulomb gauge can be interpreted as the color
dielectric function of the QCD vacuum. Furthermore the horizon condition to the
ghost form factor implies that in the infrared the QCD vacuum is a perfect
color diaelectric medium and therefore a dual superconductor. The dielectric
function is explicitly calculated within a previously developed variational
approach, using a specific ansatz for the vacuum wave functional.
