I won't invalidate your bashing but I think this was on purpose. After mushroom hill you go directly to the enemy's airbase and then deeper and deeper underground and into the unknown with the colors being more bland and sterile to help represent this. It makes the moment where you're flown to Sky Sanctuary, full of color and beauty, that much more exciting. It's a final breath of fresh air after so much grey and brown and sets you up to remember what you're fighting to stop Eggman from destroying. Because the world under him would be as bland as those scary worlds. I'm 100% sure that was a purposeful design choice.