Colours is a more difficult one to judge, they actively went with a 2D angle for the most part making a lot of the comparisons across both games here kind of skewered. But with a far greater variety of platforming sections and wisp additions I can't see it as a more automated game at all (well except that one bit in Starlight Carnival).
The rails point is a fair one as future games just made rails a way to travel from point a to b, throwing out the crouch and leaning angle with rail swapping to avoid hazards.
But the main meat of both packages is the whole running forward, homing attacking antics. Even if say Generations makes immediate small movements clunky as hell it's certainly not a sign of being stripped down, not when these actual sections tend to play a lot more varied in themselves, throwing in sonic's stomp, slide and wall jump with far greater route variation in the stages themselves which are longer as well for that matter, I don't see anything removed here, perhaps a few things streamlined for greater flow (homing onto rails and the rails themselves).
I can certainly see liking that Sonic is more easily controlled, having that level of control would be incredibly useful for the manual jumping sections of modern Crisis City in Generations but regardless I can't see this as a simplified game, especially when Adventure 2 is the one where most speed stages are narrow, linear, and lack much input from the player most of the time as far as actually progressing forward, stages like Green Forest and Metal Harbour are as simple as 3D Sonic gets really.
Don't get me wrong, while I get the angle you're coming from, say Unleashed has Sonic in a constant state of go-go-go, run, drift, jump, lock onto the swing bar, jump off towards enemy chain, homing attack times 3, move near and lock onto rail, BOOOST.
A similar chain of events in SA2 (replace drift with one of those trick ramps) would be a more slower paced affair yet the level of control over your character in the air between these moves would be greater and the difference between hitting that last rail and completely missing it. But this isn't so much the game being stripped down, as i've said earlier, streamlining seems more like a better choice of words.
