Hello Fred
Yes, it might be a little confusing if its the first time she listen that expretion.
I would rather use "Life is not all black or white", that its almost a litteral traduction of "[En la] vida no es todo blanco o negro".That sentence express the same idea as "life's subtle differences" or "shades of gray".
In spanish, your sentences would be "[las] diferencias sutiles de la vida" and "escalas/matices de gris", respectively.
I hope this answer works for you.
Regards