To change the background color of text back to "transparent" (none), you will need to do the following.  From the toolbar on the Editor, click on the "HTML" button and a pop-up will come up.  You will need to look for the text that you would like to change the background color for and modify the following attribute for this text (see below).

Example:

style="background-color: #ff0000"

change this to

style="background-color: transparent"

When done revising this attribute, click on the "Update" button at the bottom of the pop-up window and this will also close out the pop-up.  Finally, click on the "Process Page Update" button below the editor box to save your changes.

NOTE: The above will require some previous knowledge of HTML.  If you are unsure as to how to make the changes above, please let us know so that we can further assist you with this.  Thanks!