This makes the numbers go red when inserting, and blue when not.
```
function JdaInsertEnter()
  highlight LineNr ctermfg=yellow ctermbg=darkred
  highlight CursorLineNr ctermfg=black ctermbg=red
endfunction
function JdaInsertLeave()
  highlight LineNr ctermfg=yellow ctermbg=darkblue
  highlight CursorLineNr ctermfg=black ctermbg=blue
endfunction
call JdaInsertLeave()

autocmd InsertEnter * :call JdaInsertEnter()
autocmd InsertLeave * :call JdaInsertLeave()
```