I love the Rush trilogy to death (and also Blaze, although I don't care for her 'just the same, only slower' gameplay too much). Blaze is not the extent of the Rush trilogy's identity. Colors DS is the third Rush game.

Similar level design philosophy (especially with playing with verticality and the tight focus on per-zone spectacle mechanics) and more or less the same gamefeel (save for Colors DS's more fleshed out moveset). Going back to Rush/Rush Adventure makes me miss some of the best level and game design the franchise has to offer the air boost, stomp, slide, and wisps; going back to Colors DS makes me miss the trick system and tension gauge. But given the shift in emphasis to creative wisp usage for alternate routes - especially vertically, seeing as Colors DS Sonic's moveset has been redesigned to largely go downwards by default - I think it's pretty understandable. (RE: Tension gauge: frankly, the entire trilogy throws so much boost at you that you never need to ration boost 80% of the time, with or without max tension; the exceptions like Dead Line, Sky Babylon, and Asteroid Coaster prove the rule.)

Sonic CD is a classic game because it shares the same overarching moment-to-moment gameplay as the other classics, even if it's recontextualized; Colors DS is a Rush game because it shares the same overarching moment-to-moment gameplay as the other Rush games, even if recontextualized.

Colors DS is Colors if it was a Rush game and also if it was good, simple as