functions.php return custom text

You can use get_blog_details() to do it.

Use it the following way:–

function greeting() {
    global $blog_id;
    $current_blog = get_blog_details( $blog_id );

    if ( 'one' == $current_blog->blogname ) {
        echo 'hello';
    } elseif ( 'two' == $current_blog->blogname ) {
        echo 'goodbye';
    }
}