Within vi if you can't exit /etc/vim/vimrc you can type

:syntax off

And to turn it back on:

:syntax on

Many of us have a dark/black background in our terminal/shell making the default syntax unreadable, the solution is to either turn off syntax highlighting with :syntax off or to enable a special option in /etc/vim/vimrc

Edit /etc/vim/vimrc

The "syntax on" enables the highlighting permanently, and "syntax off" turn it off permanently (but of course it can temporarily be enabled or disabled within vi as shown above).

if has("syntax")
  syntax on
endif

" If using a dark background within the editing area and syntax highlighting
" turn on this option as well
set background=dark
 

If you are using a dark background then also set the "set background=dark" option on, otherwise it makes everything unreadable. I struggled with this until realizing that was an option.

You can also run "set background=dark" from vi itself.