Hi Antoniseb,
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The image shows this quasar on the edge of a region that emits a lot of light. What makes you say that this region is opaque? Also, the quasar light is coming at the edge of this region, not the center, or at the leading edge of a dusty lane.
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When a galaxy emits light, it needs stars and dust/plasma to do so, that's what makes it opaque for any background object's light. The false colors indicate different shades of opacity (at least that's what I gather from Isferno's explanation) if we were looking for a transparent part in this image enhancement, it would mean it should be white? (correct me if I'm wrong), so any other color means higher opacity.
Furthermore the reddening of objects behind galaxies is readily shown by reddening of the spectrum, which the people in the article showed isn't there, as DGR explained. The way the spectra were measured is also described, complete with spectra.
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Side note 1: this quasar's brightness may be enhanced by some weak lensing.
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The brightness should be weakened by the intervening galaxy.
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Side note 2: there would certainly have been some difficulty separating out the spectrum of this quasar from the forground stars in NGC 7319.
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See article,
Cheers.