We report a strong suppression of the three-photon ionization resonant with the 2s state of atomic deuterium at 243 nm by the introduction of a second colour (486 nm) coupling the 2s and 4p states. This coupling reduces the ionization probability by several orders of magnitude. The experiment is performed using fundamental and second harmonic frequencies of the same laser, with laser intensities up to a maximum value of and an atomic target density of the order of . The production of both ions and D(2s) atoms is measured as a function of the intensity of the fundamental beam (486 nm). The experimental data are compared with a theoretical calculation using essential states and taking into account the experimental laser bandwidths.