Are you judging by experiencing them on a CRT, or from modern displays? The range of colours was certainty broad enough, but I agree that the screenshots tend to look a bit bland. I wonder how much this came down to variance of the video signal at the analog stage. You had knobs for brightness, contrast, and saturation instead of the digital tweaking you have now. Perhaps it is that the color representation of live video has improved so that the knob level adjustment isn't needed anymore, but the output of the old consoles were tuned that level of adjustment people used to use to make the original video look nomal.

I don't know I liked how each video game console at the time had its own audio and graphical signature. We were a Sega house but we enjoyed swapping our console for a few months with the SNES or PC Engine of friends/cousins and I think they liked it too.

It might have more to do with the game studios ecosystem more than technicalities given some games were ported on all 3 consoles with only minor differencies though.

I would describe both the audio and graphical signature of the megadrive as "Metalic" (a bit amiga 500 like but with FM sounds instead of samples) while the SNES one was more childish/cartoony (same as subsequent consoles from the brand really) and the pc engine one had more flat[1] graphics but more saturated/rock'n'roll sound signature.

[1] probably because it was halfway inbetween the sega master system and megadrive/snes in term of gfx capabilities.

Specifically the SMD global palette had limitations around desaturated/pastel colours, with choice in saturated colours. And with no sprite blending, opportunities for subtle tones are further limited.

Sonic being the exception