Exporting to RTF doesn't respect background opacity settings
 

I had about 8 rows with background set to yellow. I set their background opacity to 0% since there's no other way I could see to clear a background. I then exported to RTF, and each row appeared with a yellow background.

I don't believe RTF supports opacity. You certainly can't set it through TextEdit.

As far as removing a background color, select the rows/text and look at the Style Attributes inspector. You should see the background color listed there, click the 'x' to the right of the entry to remove it.