The best thing Colors did was have no introduction. As soon as you press start the game begins.
Unleashed was a mess of a game, but I enjoy it a lot more for a lot of reasons I have seen have already been mentioned here. Some reasons not pointed out are how the controls are much better on the HD releases than Colors, which would change based on your perspective... Such a bad idea for a kids game.
Colors is the first 3D Sonic game since Sonic Adventure 2 that I believe is unarguably good. Not great, but good. Nothing in particular about it is awful or done particularly poorly whereas I enjoy Unleashed more, I could talk about it's issues for hours.
But like with pretty much all side games, Colors is fairly forgettable (notice how people never talk about it anymore already) and just does not share a similar scope to the other releases. Sonic Unleashed was the last platformer I had played for months just because there was so much content overall. If you get rid of the frustration, add in better level design (some of the level mechanics used in every Sonic games for years just are not working and need to go) and bring in rolling and art direction similar to Sonic Adventure 1 and the original trilogy, I would really be happy, even if I know it could never be the same quality of the games I just mentioned.
