I was trying to get the value of a color picker, where I needed to pass the ID of the post.
The following did not work, even though I'm using this technique to get other meta fields from this particular page.
$background = rwmb_meta('guide_color_picker', $child->ID);
I had to pass an empty array to the $args field in order to get the value. Otherwise I was getting an empty stting.
$background = rwmb_meta('guide_color_picker', array(), $child->ID);