I believe that highlighting is just letting you know you have some inline css in a non-editable region..
I don't know if it's a "bug" or a "feature" but my copy of DWCS4 does the same thing..
Everything validates fine Nancy, in both the template and child pages, so I don't think it is a serious error, just Dreamweaver trying to tell me something. I spent a whole afternoon trying to discover what that might be, but could find nothing.
Seems you are right Jon that it is just telling me about inline css, because that appears to be the only thing that is affected. I never realized before that my original template must have been entirely free of inline css and that is why I did not see the phenomenon until recently. When I started out I thought you put all css stuff in the separate css documents, but as I learned to use inline css I of course started using it more and more and as it made its way into template edits the new red and blue highlighting started to appear in all the dependent pages. Of course my first thoughts were always going to be that it must be some kind of error message.
On reflection it is probably more efficient to use style sheets for master templates and the code that will be common to hundreds of pages, so I did a little editing to relocate the inline css and the highlighting has gone. Perhaps that is what Dreamweaver was trying to tell me.