I have an odd field with a purple background, mentioning HTML-codes to use while leaving comments

This page says that feature was removed from WordPress a long time ago, so you may have a theme or plugin which has put it back.

You can try using the same filter to remove it again. To use the filter you need to add this to your functions.php file:

function remove_comment_prompt( $defaults ) {
    return "";
}

add_filter( 'comment_form_defaults', 'remove_comment_prompt', 999, 1);