I tested this on the TinyMCE demo site and couldn't reproduce it. So this is either fixed in a new version or it's a bug specific to Confluence.

1. Write some text in the editor
2. on some of the words apply a different foreground colour (like red).
3. now select a few words including some words with the default black text and some words with the red text.
4. delete the section.
5. the red text on the remaining red letter is removed.

perhaps a video desribes it better: http://screencast.com/t/nLeiGf9vrUy