Can they port Sonic Colors stages over?
Once they figure out the formats for everything, more than likely.
But the only reason they're doing Sonic Unleashed ports right now is because it's the same engine. A lot of the data formats are similar that it's just a matter of cleaning up/converting the Unleashed stuff.
Porting over levels from Colors (or any other game) will probably require a lot more work. New lighting data will have to be generated, scripts for camera controls and such will probably have to be re-written, and a lot of level objects will probably have to be replaced.
The lighting data will probably be the hardest part. Baked Global Illumination lighting (as used by Hedgehog Engine, Unreal Engine 3, etc.) is massively processor intensive; most companies calculate GI lighting data using networked render farms (like the ones used to render Pixar movies), and going by what Sonic Team said, it took
days to render GI for a single Sonic Unleashed level, and their render farm was apparently 100 PCs strong.
Since Sonic Colors doesn't use GI, new GI data will have to be generated if it's to be used in the Sonic Generations engine. That time may be reduced depending on the format; assuming Hedgehog Engine's GI data functions like a normal lightmap, you could skip full GI baking and just do more simple colored lighting (the kind Sonic Colors already uses), which would bring rendering times down to something more manageable.
If this is all going over your head, I'll explain it to the best of my knowledge, though the last time I did this I got yelled at for getting it wrong (even though I haven't actually seen any evidence since then to suggest I'm incorrect):
Starting with Half-Life or Quake 2 or so, the concept of "lightmaps" have been employed to apply lighting to a 3D environment. Imagine a 3D model with a transparent texture overlapped across it; this texture is black and white, and represents areas of light and areas of shadow, giving the 3D model the appearance of having lighting.
Here's a comparison I found on Google Image Search that shows a scene progress through wireframe, textures, lightmapping, etc.
Hedgehog Engine's GI data is like a big lightmap, but the way the lightmap is created is far more complex; rather than simply having black-or-white shadows, Hedgehog Engine looks at all the colors in a scene and simulates light bouncing off of objects realistically. So, light shown on a blue surface will reflect blue light on surrounding objects.
Here's an example given at the initial technology presentation. And here's an image showing the whole thing as rendered by Sonic Unleashed:
Games like Sonic Colors for the most part only render basic lighting, or use some other less-complex method of lighting.
tldr: Yeah probably, eventually.