Hi!
Once you bring the picture into Photoshop, it's no longer a vector image like the artwork that you create in Illustrator. Once it's in Photoshop, it's a pixel image and there's no going back to vector. So, even if you bring the image back into Illustrator, you can't edit it like you did originally.And, yes, it will be pixelated.
The best thing that you can do in this case is to leave it in Illustrator and color it there. For the most part, you can color things just as well in Illustrator as you can in Photoshop, so there's really no reason to take artwork over to Photoshop to color it. You can change colors in Illustrator by changing the fill and stroke attributes of the vector object. You can fill objects with a solid color, a gradient, or a pattern, just like in Photoshop. It's not hard to do once you understand what's going on.
I searched around and found a video that might help:
http://www.youtube.com/watch?v=CBiLwipwXng
Hope this helps!
Good luck!
Mary
http://www.ScrappyDigits.com