ENDBG is a rendering shortcut settings predefined in TWikiPreferences. See the section rendering shortcut settings in that topic for a complete list of background colors.
%REDBG% red background %ENDBG%
%<color>BG% section must end with %ENDBG%. If you want to switch from one background color to another one you first need to end the active background color with %ENDBG%, such as %REDBG% some text %ENDBG% %GREENBG% more text %ENDBG%.