I guess the other thing is that as a tribute game, Generations by its very nature lacked freshness and originality, good as it was.
Colours on the other hand had cool new settings like Planet Wisp, and it was just more exciting feeling like "I wonder what new things the next level will bring" instead of "I wonder which old level will be remixed next?"

Colors, it has simpler level design, but at least remains consistently fun and compact through its runtime, with a good amount of creativity in its level themes, plus decent writing and a fantastic presentation, both visually and musically. Is not so much about going fast, but that is usually an advantage given the more fair level design for the most part.

Generations has a lot of bumps, namely the missions being tedious for the majority of them and it runs out of steam in the end, with a terrible final level and final boss, making the end note of the experience feel somewhat sour, feeling incomplete in terms of its cut scenes and the like, and the visuals looking inferior to Colors and Unleashed in my opinion.

Colors on the whole is the best Sonic game.

Generations. I liked the nostalgia trip, and Colours just felt rather "meh" to me, for what time I got to play it at a friends.

Also, Colours contains one of the greatest and most epic pieces of music in any game, ever.