Yeah, colors are more muted, but that's how they look on the art print. And that one was most likely supervised by Steve Purcell himself. Also the colors on the boxes are overly contrasty and generally quite inconsistent.
I finally had the courage to take my "Sam & Max Hit the Road" print out of its frame and scan it. All three posters have been replaced in the main post. The upgrade in quality is quite extreme, so here are a few comparisons at full size: