When I write a big text exceeding the window bounds when starting Emacs the fonts appear in other color than the rest. If it makes any good to mention, when I disable font-lock mode text appears in one color as it is.
M-x customize-group RET whitespace RET to customize how
whitespace-mode works. In particular, you can scroll down to the "Whitespace Style" setting, and uncheck the "Lines, only overlong part" option to turn off this particular effect. Use
C-x C-s to save the settings, and then reload or revert the buffer to update it.