The phenomenon of color confinement has yet to be derived from Quantum chromodynamics, but it may roughly approximated by the phenomenologically successful bag model. In this model the normal vacuum is a perfect color magnetic conductor, that is the color magnetic permeability k is infinite, while the vacuum in the interior of the bag is characterized by k=1. This implies that the color electric and magnetic fields are confined to the interior of the bag. Now even in an “empty” bag, there will be nonzero fields present because of quantum fluctuations. This gives rise to a zero-point or Casimir energy, as we have seen.
The Casimir Effect by K.A. Milton p105