i want different colour for submenu text

i want the submenu text colour to be white , while the mainmenu text colour another colour, when i go to Apperance - customize - Header- Header text/Menu text , the colour overrides that of the submenu text colour, pls how do i fix this. secondly i tried changing the text "curriculum" to course module in the language file, its not reflecting, am i editing the wrong language file, pls do guide me. i edited en_US.po