You can save your newly edited sequence from Final Cut as XML. Then open the original Color project and choose "reconform" from the file menu, select the XML file when prompted, and it should update your color project with all the new edits. Works most of the time, at least in the most recent version (I remember it being a little buggy in the first version of color).
Also, if you just resend the sequence as you described and only color correct the new clips, then those should be the only ones you need to render in Color. In that case the other clips will be exactly the same when you go back to FCP with no loss in quality.