  The vacuum dynamics of SU(2) and SU(3) lattice gauge theories is studied by
means of a gauge-invariant effective action defined using the lattice
Schr\"odinger functional at finite temperature. In the case of the SU(3) gauge
theory numerical simulations are performed both at zero and finite temperature.
The vacuum is probed using an external constant Abelian chromomagnetic field.
At zero temperature, in agreement with our previous studies for the SU(2)
theory, the external field is totally screened in the continuum limit. At
finite temperature numerical data suggest that confinement is restored by
increasing the strength of the applied field.
